Apps, Widgets and Plugins - Windows
Triqqr - Submit tweets directly to Twitter and view user profiles.
Pwytter - Cross platform Python Twitter client with Asian character support.
Teletwitter - Experimental open source client.
TwitBox - View and submit tweets, see replies and direct messages, delete your own tweets, and multiple account support.
Twitterlicious - Update Twitter status, easily check replies and direct messages.
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.
TwittIt - Extremely compact and simple app for submitting tweets.
MadTwitter - A reincarnation of Twitterrific, except on Windows.
Twitter CLI - Post tweets via the command line.
What’s Up? - Gadget that allows you to see latest tweets from your friends.
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.
Search
Twitterverse - Search through archived public timelines and tweets.
Twittersearch - Search Twitter and tweets by word.
Terraminds - Search for specific users or tweets.
TwitDir - Allows user to search for users by name, location, or username.
Browser Plugins
TwitBin - Send and view tweets right within Firefox.
TwittyTunes - A FoxyTunes addon that posts what you’re currently listening to Twitter.
Shareaholic - Share links and webpages via Twitter right within Firefox.
Search Plugin - Tweet right from the Firefox search box.
Tweetbar - Put Twitter on your Firefox sidebar.
Twippera - Twitter widget for Opera that can send and view tweets.
TwitterFox - Compact Firefox extension that can publish and view tweets.
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.
Из Mashable