All Platforms  

Web  

EntourageBox - Let Friends Upload Files to your Dropbox.

Backdrop - Automatically copy Basecamp files to Dropbox.

Backup Box - Automatic scheduled transfers between your FTP or SFTP servers, Box.com, MySQL and other databases, Microsoft SkyDrive, Google Drive, and Dropbox accounts.

Hojoki - See and comment on updates in your dropbox along with other apps like Evernote and Google Docs

foneFrame - host your mobile web pages in your Dropbox Public folder

KISSr - Enables you to host websites out of your Dropbox

CloudHQ - Back up Google Docs and Basecamp to DropBox

Otixo - Cut, copy, and paste files between Dropbox, Google Docs, SugarSync, FTP, and WebDAV. 

Fuzli - Build your own Fancy Photos Gallery from your Drobox folder (iPhone & iPad compatible). 

Socialisr - Allows users to store Dropbox photos and share it with friends 

AirDropper - A web application that allows Dropbox users to request a file from anyone, even if the person doesn't have a Dropbox account. 

JSBox - Allows you to edit text files or restore sets of deleted files directly from your dropbox interface. 

Dropbox Forms - Get people to send files from your web site to your dropbox. 

FileStork - A web app that enables Dropbox users to create one time or persistent file upload forms that allow others to upload files directly into your Dropbox. No longer available.

300milligrams - Allows you to comment Dropbox files.

BusyFlow - Integrating dashboard for cloud-based collaboration and productivity apps.

Dropbox Gallery RSS - The dropbox photo gallery RSS feeds are currently (Nov 2011) broken, this mini-app provides a working solution.

Dropbox Automator - IFTTT for Dropbox. Takes actions based on files being saved in to your dropbox folder(s).

Userscript (Greasemonkey)  

Yahoo! Mail Dropbox Tab - A userscript to add a new permanent Dropbox tab in the new Yahoo! Mail. (Tested in Chrome and Firefox + GreaseMonkey). 

Dropbox Plus - A userscript which adds some improvements to Dropbox interface (treeview of the folders, button to empty the search box, file list with scroll bars, etc.)

PHP  

My PHP DropBox Gallery - A PHP script to generate a photo gallery on the fly on your server, from a directory of your dropbox. 

PHP file checking script - Check for invalid UTF-8, invalid ASCII, and invalid windows filename characters. As well as getting a count of files & folders. 

PHP Dropbox Uploader - You can use this to add a file upload form to your webpage which uploads files to your Dropbox. 

A PHP script to move your Dropbox location. 

PHP Email To Upload - A modification of the Dropbox Uploader to check an email address for attachments and uploading them to your Dropbox. Handy for iPhone uploads. 

Coldfusion  

http://dropbox.riaforge.org - Easy to implement solution that doesn't require any server-side setup changes to access (ie. no SSL cert installations, etc). 

Python  

Uniform reference to shared files - This script provides a way to reference shared files across accounts and clients. 

pyDropboxPath - Script to relocate Dropbox folder (once known as DropboxPath but renamed to avoid conflicting with DropboxPath). 

Python Dropbox Uploader - Similar to the PHP Dropbox Uploader, but in Python. 

A Python script to display Dropbox client configuration data. 

An improved python script to display dropbox configuration data - works on 0.7 and 0.8. 

A python script to save and restore dropbox proxy settings.

Perl  

http://search.cpan.org/dist/Net-Dropbox-API/ - A dropbox API interface 

https://github.com/s-aska/dropbox-api-command - Dropbox API wrapper command. ( ls, find, sync, cp, mv, mkdir, rm, get, put ) 

Other  

Nagios check - A nagios plugin to report Dropbox status. 

The dropbox plugin - You can use this plugin to integrate Dropbox into Wordpress to download, browse and upload files dropboxConnections.php - Dropbox Class. 

Joomla Component - A ready to use Joomla Component for showing a Dropbox directory or view an upload form. 

Drupal Module - A module which allows users to upload files from a Drupal website into an existing Dropbox account. 

Contao (fka Typolight) Extension - a module which allows extension authors to integrate Dropbox into their extensions, for example for downloading files. 

Ruby Dropbox API - A ruby script for interacting (List, Upload, Download, Delete, Move, History) with a Dropbox. (gem install dropbox).

Dropbox Gallery Download - small bash script using wget to download all photos in a public dropbox photo gallery. Updated with PHP and Perl translations.

DropDiff - diff the two most recent versions of a file in your dropbox folder. 

DropboxDiff - a Google Chrome extension which triggers your custom diff tool to compare versions of your Dropbox files. 

GetDropboxFolder - a simple bash script (containing a reusable function) to get the Dropbox folder location.

fake_dropbox - A simple fake implementation of the Dropbox API written using the Sinatra framework (in Ruby). For development and testing purposes, no real authentication and users. When you run it, it listens on a selected port, serving files from a local directory, just like Dropbox API would.

dummy_dropbox_sdk - Very simple library for mocking the dropbox-ruby-sdk by redirecting all calls to dropbox to a local folder.

Shadow - Backup and automatically restore files/folders deleted from shared folders. 

Mac OS X  

DropClip - Send coworkers directly to any shared Dropbox file on their Mac. DropClip creates a clickable link that opens a Finder Window and highlights the shared file on their local hard drive.

[Spotdox] - See all the files on your Mac from any phone or web browser, remotely upload any file from your Mac to your Dropbox. Its like having a TB Dropbox for free!.

Quicksand - A free Mac app for syncing your recently opened files with Dropbox and other cloud services.

MacDropUpdate - An automatic updater that updates you every night to the latest Dropbox forum build. 

MacDropAny - A simple, painless way to sync any folder on your computer with Dropbox! 

Broomstick - Hide your Dropbox menubar icon with just a single click! 

DropLink - Quick and easy application to synchronize any file/folder of your Mac! Place into your Dock and forget any pain! Work with Symlinks & Growl. 

Dropbox Dashboard Widget - An OS X Dashboard widget that will easily allow you to drag any file(s) onto the widget and copies the dragged files to your Dropbox Public folder and copies the URL to easily share the file on the internet with others. 

Dropbox Droplet - A little application sitting in your dock, waiting for files to be dropped on it. The dropped file is then uploaded to your Public folder and the corresponding URL gets copied to your clipboard. 

Dropbox Service - Contextual menu entry to copy or move files into your public folder and put the public links in the clipboard. 

DropShorty - a Cocoa-based droplet that integrates your Dropbox account's Public folder to generate short URLs from Dropbox's somewhat long URL scheme. 

Dropbox Encore - enables a second instance of Dropbox to run in a single Mac OS X user account.

Bloodrop - droplet (Dock icon) for Mac OS X (AppleScript application). Drag file(s) to Bloodrop icon in Dock and paste public Dropbox link(s) from clipboard. Easy per user interactive configuration, Growl notifications. 

Windows  

Click2Public Dropbox Addon - A lightweight free utility that turns your Dropbox to handy file sharing service. It adds context menus to Windows Explorer, allowing you to copy or move files to your Public Dropbox folder with the one mouse click. Then places the URL pointing to your newly uploaded file right into the Windows clipboard, allowing you to paste the link into a blog post, Skype conversation or comment right away.

Cameyo App Virtualization - Use your apps directly from Dropbox. Turn any Windows application into a single file that can run directly from your Dropbox folder. Keep your app's state, settings & data consistent between your computers. 

CloudZap - Print, Scan, and Publish to Dropbox. Use SnagIt and CloudZap and publish directly to your Dropbox. 

Dropbox Shell Tools - A lightweight utility to add dropbox to windows shell menu. (from Perry Wolf - actual for win xp/vista/7 as ver.0.2RC1). Note: Deutsch Only. No longer available.

Dropbox Buddy Desktop Suite - A powerful, full featured Windows client/server .net desktop application that turns Dropbox into a "2-way street". Now you may download files and control a remote server machine from another (client) computer, Start and end processes, run batch commands and get lists of running processes. Latest version 1.7 - 17/01/2011. 

Dropbox Buddy Client Portable Installer - A portable apps installer for the Dropbox Buddy client application. Latest version 1.6 (now upgraded to SP1 - 30/09/2010). 

Dropbox Folder Sync - Sync (and Unsync) any folder to Dropbox with just a right click. The application moves the selected folder to the dropbox folder and creates symbolic link at the original location thus effectively viewing the same copy of files at both locations. Useful for syncing firefox profile, Saved games etc. 

Akira 1.0 - A lightweight utility that uses Dropbox to send commands from one PC to another. 

OneSync 3 - A free and open-source sync tool that maximizes lightweight synchronization experience and provides both offline synchornization through thumb drive and online synchronization through Dropbox. The latest version is 3.0. 

Dropboxifier - A simple application to help you create and manage Dropbox links across multiple machines. Requires Windows Vista or higher and .NET 4.0. 

Dropbox Screen Grabber - A simple application which sits in the system tray and allows you to capture the screenshot of the whole desktop or the currently active window and save it to the Dropbox public folder. 

Dropbox Uploader and Screenshot Utility - Application that allows you to take a screenshot of a region of the screen and saves it in the Public folder as well as copying the URL to the clipboard. Also includes a drop zone that allows you to drag and drop files to upload.

DropboxPlugin - This program helps you share your Dropbox Public files with the one mouse click. The application copies and moves files to the Public folder and retrieves all public URLs and places them on to your clipboard. This plug-in has a friendly interface and is simple to use. Now you can move your 'My Dropbox' folder using this plug-in. You don't need to install any libs or frameworks to run this software. Lastest Version is 1.5.1 03/03/2011 

Doc Beautifier for Dropbox - A simple addon that watches your Dropbox folder, and automatically converts any .doc file to PDF which has been created, renamed or modified (all existing .doc files will be converted during setup). This will give optimal viewing experience for the iPhone/iPod Touch/iPad. 

DropBox Portable ICE 0.5 - Makes Dropbox compatible with any USB flash drive (compatible with 0.8.X versions of Dropbox) and has some nice features. Because it requires .NET Framework 3.5 it will not work on most of public PCs with installed Windows XP but only on Vista and 7. Discontinued. Try using app beneath for fully portable and up-to-date Dropbox. 

DropboxPortableAHK - Fully portable launcher for Dropbox, written in Autohotkey . Thanks to built-in updater, it will always be up-to-date. 

Dropboxen - A multiple Dropbox launcher. 

DropboxPath - Allows you to set the path to your Dropbox without any restrictions (so it can be placed on removable drives). 

DropboxDataWrapper - Allows you to change the location of the Dropbox data files and cache. 

A Python script to get the Dropbox status of a file or folder in Windows 

Conflict Manager - A Windows application that allows you to manage conflicted files and choose which one to keep. 

Public Link Automation - A Python script that copies selected files via the "Send to" right click menu directly to the public folder or a subfolder, and places the corresponding public links in the clipboard. 

MailDrop - Email files to your Dropbox. 

FirefoxProfileCleaner - A simple application that removes all conflicted copies of files from your portable Mozilla Firefox profile. 

ProxyOnOff Simple scripts to stop Dropbox, change proxy settings, then restart Dropbox. 

An EXE to get the Dropbox status of a file or folder in Windows 

BATCH Dropbox Uploader for CMD.EXE - Batch script which can be used to upload files to Dropbox from command line. 

Dropbox Connector for eCopy Sharescan - A connector for eCopy Sharescan which can be used to scan documents in many format from most Multi Functional Devices, including Canon, Xerox, HP and Konica Minolta printers.

ReturnDropboxStatus - A dll that provides information what Dropbox is doing. eg. syncronizing or idle or offline.

Conflicted Copy Pro - Conflicted Copy Pro is a Office Add-in that prevents Dropbox and Dropbox Team users from opening the same Word, Excel or PowerProint shared file at the same time.

Linux  

DropFuse - Linkable folder filesystem. Mount a linkable folder and use it in a regular file browser. 

Linux CLI - A CLI for the Linux Dropbox client. Supports almost all commands available in the GUI. 

A Ruby script to continually log events from the Dropbox client in Linux 

A Python script to ascertain the current status of the Dropbox client in Linux 

A Python script to make fake GUI libraries for the Linux Dropbox daemon 

Delay startup and run dropbox at idle IO priority 

Dropbox experimental build update script (a.k.a. dbupdate) 

Dropbox in a computer... without the computer! 

KFilebox is an application which offers almost the same features as the official Dropbox Linux application, but works on KDE. 

Dropbox integration for Dolphin - A Dolphin (KDE file manager) plugin to provide overlay icons and context menus in KDE

Thunar Dropbox - A plugin for Thunar (on Xfce) that adds context-menu items from dropbox 

BASH Dropbox Uploader - Shell script which can be used to upload files to Dropbox from command line. 

drop - A shell script which can be used to upload files to the Dropbox public folder and get the public URL automatically. 

A python script to enable/disable LAN Sync 

Dropbox Share - A Nautilus script to upload files or folders to the Dropbox public folder via right click (using symbolic links) and get the public URL as well as unshare those files. It also adds an emblem on the shared file, uses NotifyOSD to notify the user when it's ready and clears removed shared files automatically. 

PythonDownloader - A python script to download a folder from dropbox. Utility for headless environments (bash).

Android  

D4 - 'Droid Dropbox Directory Downloader - facilitates downloads of entire folders from the Android app, because it currently only supports downloading single files. 

iPad  

Website Maker - Enables you to create web pages on your iPad and host the pages using Dropbox.

iPhone  

DropIt - Upload pictures straight from an iPhone/iPod Touch camera to Dropbox.