Desktop Clients
Twhirl – Twhirl is probably the most popular desktop client. It also supports TwitPic, TweetScan, FriendFeed and cross posting to Pownce and Jaiku.
Spaz – Desktop client for Mac, Windows and Linux
Tweetr – Allows you to send files to other Tweeters. It also has a built in webcam feature.
Alert Thingy – Keeps you updated on FriendFeed and Twitter. Allows you to easy upload pictures to Flickr.
TweetDeck – Better organize your Twitter friends with a great built in group feature.
Twitteroo – Displays updates without pictures (for better or worse).
Witty – Open source client for Vista and XP
bTT – Non-Adobe AIR based desktop client.
Twitterlicious – Minimalist desktop client.
Snitter – Customizable desktop client for Mac and Windows.
Feedalizr – Post to FriendFeed, Twitter, Jaiku and Flickr with this desktop client.
Posty – Has support for several microblogging services
Firefox Plugins
twitbin – Creates a sidebar for Firefox to keep you updated.
TwitterBar – Post updates from your address bar.
TwitterFox – Puts an icon on your bottom bar that gives you notifications and allows you to update.
Firefox Search Plugin – Post updates from your search bar.
TwittyTunes – Show people what you are listening to.
Online Services
Twitterfeed – Update Twitter with new posts from your blog.
Brightkite – Brightkite updates can be synced to update Twitter as well.
Twitio.us – Twitter + Del.icio.us
Twitthat – Bookmarklet that lets you Twit about the web pages you are viewing.
HelloTxt – Update all your services in one place
Ping.fm – Updates multiple mircoblogging services
PingTwitter – Send updates when your blog updates
Socialthing! – Lifestream plus ability to update.
Sendible – Organize your online life.