TweetDeck is great for serious Twitterers: A desktop application that lets you split your main feed into topic or group specific columns. Also has filter options and auto updates.   Related posts: Turn Web Apps Into Desktop Mac Applications Join Twibes Twitter Groups Who’s The Ultimate Twitterholic?

Tweetree: View your tweets’ replies easily, including external content like TwitPic and Flickr photos, YouTube videos and more without having to click through links.   Related posts: Make Twitter Better External Link WordPress Plugin YouTube to MP3 Converter

With Mr.Tweet you can find people on Twitter who are interested in specific topics and categories.   Related posts: Join Twibes Twitter Groups Twitter Multiple Account Manager Verify Your New Twitter Followers

Created by the guys over at Blogstorm.co.uk, Twitition lets you create, sign and share petitions with people on Twitter. As of December 2010, Twitition.com had:   – 388,000 followers – 1,996,816 signatures (almost 2 million!) – 59,164 petitions on the site – 830,000 monthly visitors -2.1 million monthly page views