Tweetscan - you can search all public tweets by keyword
Twitterverse - shows a tag cloud of most used keywords - current or in the past 5 or 10 hours
Twistori - very cute site where you can choose to see a real-time stream of tweets that include the keywords "love", "hate", "think", "believe", "feel", or "wish".
Tweetburner - go here to shorten a link's URL and then - so cool! - track how many people click it
Twit This - easy button that can be added to any page so that someone can tweet the link to it.
Twitterholic - shows updated list of most followed accounts and their stats
Twittervision - world map with tweets displayed real-time, by location
Qwitter - sign up to get email notifications when someone stops following you (if you dare...)
Also, this error message from yesterday really resonated with me:
