This, far from exhaustive list highlights some of the more powerful add-ons to Twitter and helps show the range of functions available.

• TwitSig - display your current twitter status as an image in your Forum signatures.
• TwitterBox - send tweets from inside SecondLife.
• Zannel - photosharing website that cross-posts to Twitter and Flickr  (Update - Flickr now posts to Twitter, just add it as a blog)
• Twitterfeed - take any RSS feed(s) and point it to Twitter
• Twittershare - Share pictures, music, video and other files

Resources

• FriendOrFollow - "Who are you following that's not following you back? Who's following you that you're not following back? Find out!"
• favotter - see which of your tweets have been favourited and by whom
• Tweet Friends - find mutual friends between to twitter users