ActionScript/Flash

• TwitterScript by Michael Galpin. AS3 API, originally from Twitter, now open sourced.
• SWX Twitter API by Aral Balkan.

C++

• QTwitLib by Bradley Lackey & Maks Zolin. C++ library for Linux, Windows, and Mac.  Currently in beta.

C#/.NET

• Yedda Twitter Library by Yedda. Every Twitter API method has an equivalent .NET method in this wrapper library.
• TwitterooCore API by Eric Willis/RareEdge Design Group. Binary .NET library that can be used in any .NET project.
• Twitterizer originally by DigitallyBorn, but now open source. Written for .NET 2.0.
• tweet# by Daniel Crenna. “100% coverage of the REST and Search APIs”.

Java

• Twitter4J by Yusuke Yamamoto.  BSD licensed and Maven aware pure java interface for the Twitter API.
• java-twitter by DeWitt Clinton.  Pure java interface for the Twitter API.
• jtwitter by Daniel Winterstein. Open-source pure Java Interface to Twitter.

Objective-C/Cocoa

• MGTwitterEngine by Matt Gemmell.  Integrate Twitter support into your Cocoa app.

Perl

• App::Tweet by Joshua McAdams.  Tweet on Twitter from the command line.
• Net::Twitter by Chris Thompson.  Perl interface to Twitter.
• Twitter::Shell by Daisuke Maki.  Twitter from your shell.

PHP

• My Twitter by Andres Scheffer. Set and retrieve a Twitter user’s status.
• Twitter by Felix Oghina.  Update the status of a Twitter user.
• TwitterLibPHP by Justin Poliey.  Manipulate a Twitter user account.
• Arc90_Service_Twitter by Matt Williams.  Open source Twitter API client.
• PHP Twitter by Aaron Brazell, original code from David Billingham.
• PEAR Services_Twitter by Joe Stump and David Jean Louis.
• PHP Twitter by Tijs Verkoyen. BSD Licensed.

PL/SQL

• OraTweet by Noel Portugal. PL/SQL procedures to get friends timeline and post updates.

Python

• python-twitter by DeWitt Clinton. This library provides a pure Python interface for the Twitter API.

• python-twyt by Andrew Price. BSD licensed Twitter API interface library and command line client.
• twitty-twister by Dustin Sallings.  A Twisted interface to Twitter.

Ruby

• Twitter4R by Susan Potter.  Open-source Ruby library for the Twitter REST API.
• Twittery by Chris Ledet.  Lightweight class for Twitter’s API.
• Twitter by John Nunemaker.  Command line twits and an api wrapper using Hpricot.