Twitter add-ons.

Apps, Widgets and Plugins - Windows

Pwytter - Cross platform Python Twitter client with Asian character support.

Twitteroo - Client with rurl URL shortening, clickable links and tweets, customization, and both public and friend timelines.

Chirrup - Twitter client with extensive Japanese language support. This sites in Japanese.

MadTwitter - A reincarnation of Twitterrific, except on Windows.

Twitter CLI - Post tweets via the command line for those times when an interface is just too much.

Twadget - Simple gadget that lets you view and submit tweets right from Vista’s Sidebar.

KipFolio Widget - Simple Twitter widget for KipFolio.

Twitter-Sync for Yahoo - Syncs Twitter status with Yahoo Messenger status.

TwitterYM - Simple Yahoo Messenger status updater.

Twessenger - Updates Live Messenger status to reflect latest tweet.

Twit4Live - Set status message as latest tweet, send tweets from within Live Messenger, and send tweets to contacts.

Apps, Widgets and Plugins - Mac

Twitgit - Read your friend’s tweets and submit your own, right in Dashboard.

Twitterlex - Dashboard widget that shows your friend’s latest tweets as well as allows you to submit tweets.

Twidget - Update your Twitter status within Dashboard.

TikiTwit - Match your iChat status to your last tweet.

Cross-platform Apps, Widgets and Plugins

Tweetr - Submit tweets, upload files, urltea built in, delete messages.

Spaz - Cross-platform client built with AIR.

Tweeter - Java based client with customizable UI.

Twinja - AIR based client which allows you to follow users on the fly.

Search and Directories

Twittersearch - Search Twitter and tweets by word.

Twellow –Billed as “The Twitter Yellow Pages”  the first of several Twitter directories.

Browser Plugins

TwitBin Twitbin is a firefox extension that allows you to keep up with all of your Twitter conversations right from your browser sidebar.

TwittyTunes - A FoxyTunes addon that posts what you’re currently listening to Twitter.

Shareaholic - Share links and webpages via Twitter right within Firefox.

Tweetbar - Put Twitter on your Firefox sidebar.

Mobile

iTweet - A mobile Twitter site designed for the iPhone.

iTwit - Mobile version of Twitter made for the iPhone.

Tweeter - Twitter on your mobile phone while avoiding text message charges.

Jargong - Browse Flickr, do some social networking, but most importantly, check and publish tweets.

WildSets - Extensve Twitter app for mobile phones.

TwitterBerry - Twitter built for various BlackBerry devices. Update and view timelines with ease.

jTwitter - View timelines, enter updates, and more, all on your mobile phone.

TreoTwit - Easily check and update your Twitter right from your Treo.

Random/Other

Twittervision - See where in the world people are submitting tweets from, real time.

TwitThis - Allow visitors to your site to easily share a page or post via Twitter.

MoniTwitter - Receive important updates about your site via Twitter.

TwitterNotes - Share private or public notes via Twitter.

RSS2Twitter - Share any RSS feed on Twitter, and have new RSS items automatically be tweeted.

TwitterBuzz - Find out what is being most linked to on Twitter.

Twitterholic - Shows the top users and accounts on Twitter.