Start with the very essential - twitterfeed, feeding your RRS directly to twitter. The link would be displayed in tinyurl mode. The new best way to let your friends see your blog update!
TwitterFountain: Basicly springs out the twitters in a live motion, quite interesting at first sight, but then as you realize there are so many languages in the world you don't understand... kinda discouraging...
tweetVOLUME: Similar to Google Fight, enter a few keywords and see their appearance frequency on Twitter.
Twitterment: A yet more detailed review of your keywords on Twitter, including recent twitter messages, hourly and daily trends, and a Google Map graphic of the whereabouts of users who are using the keywords, other buzzwords and so on. Developed by eBiquity Research Group, University of Maryland Baltimore County (UMBC)
TwitterFox: a Firefox extension that works like Twitterific, lets you keep track of your friends' update and send quick twits as well, all from a tiny icon from the browser status bar. [Download TwitterFox Firefox extension]
twitigg: a website truly live up to its name: a meshup of twitter and digg. It picks up the links and videos people put up in their twitter status. Don't know whether to call it a genius idea or a failure - the website is just painfully ugly, it actually bothers to seperate "popular videos" from "popular movies", oh man. And maybe because this site is premature and right now really not a whole bunch of people are using it, it's kinda sad to see even the most "popular" only has about 5 to 6 twitters...
Twitter Mail: it lets you update your Twitter status by sending an email to a address it creates for you. An alternative, yes, but a bad one. Why would anyone do that. If you've got Internet and a browser or whatever you use to send the email, you could update your status by web or by IM and everything. If you send the email by your cell phone, and it all makes more sense to just update Twitter by sending SMS. Why bother coming up with so much useless stuff when Twitter has already provided you with so many alternatives.