Personally, I think this is the best Twitter client/interface I’ve used so far – they also have a version available for the iPhone.
Find out more about your Tweeting-patterns:
Find out about your Twitter ‘influence’, ‘signal’, ‘generosity’, ‘velocity’ and ‘clout’:
This is interesting – build a psychological profile of a person based on the content of their tweets:
Find out who isn’t following you back and who you’re not following:
Lets you search for links on twitter (even through URL shorteners). Useful if you want to see who’s been tweeting about your website. They also have a WordPress plugin for it.
Twuffer is a “Twitter Buffer” that allows you to schedule tweets for a later time
Qwitter lets you know when someone has stopped following you
Did I miss something? Let me know in the comments. You can also follow me on twitter