• LazyTweet - be lazy, ask the crowd
• Add Tweets - a free service  that lets you add your latest tweets to your blog or website using a small piece of JavaScript code.
• Twellow - an automatically generated directory of Twitter users, organized by occupation
• Monitter - It's a twitter monitor, it lets you "monitter" the twitter world for a set of 3 keywords and watch what people are saying.
• ToAnswer - a Twitter-based mini-application that will help you get your questions answered quickly.
• Tweetbeep - set up alerts on twitter messages
• Twitturls - People post links in their twitter feed. They grab 'em, visit 'em, count 'em, score 'em and list 'em.
• Feed Tweeter - Link Twitter and Plurk to your other activities
• TwitterFeed - Send your blog feed to your Twitter account
• EasyTweets - The multi-account microblogging platform
• The Mattinator - manage multiple Twitter accounts
• TwitKit - Firefox extension, Twitter appears in a sidebar
• TwitterFox - Firefox extension notifies you of your friends' tweets
• TwittEarth - Live twits from all over the earth
• Twittercount - communicate and track the amount of followers you have on Twitter
• Tweetcube - share files on Twitter

• BeTwittered - iGoogle gadget. Refreshes every 3 minutes, and you can post updates without ever leaving your Google home page.
• Twhirl - this is a desktop client so you don't have to keep going to the website to send your tweets
• LiveTwitting is a new and easy way to cover conference sessions!
• TwitterPhone - Send messages to Twitter using voice
• Timer is a Twitter bot that helps you set an alarm for things you need to remember.
• Facebook Twitter application: Do you use Facebook? Add your Twitter account so that it updates what you are doing in Facebook, and use Twitter from within Facebook
• @TrackThis - Track your packages over Twitter
• TwittEarth - Live twits from all over the earth
• GroupTweet - send private Twitter messages to specific groups of friends
• Twit This - an easy way for people to send Twitter messages about your blog post or website
• Quotably - Reformats twitter messages into threaded conversations
• twitabit - Post your tweets here when twitter is down — and they'll queue them up and make sure they get delivered.

For more see: Twitter Fan Wiki