Tweeter can be found at www.tweetdeck.com
If you have loads of followers, a good site is http://twittgroups.com/ If you join this site you can create, join, or post to groups on Twitter.
A good place to track your tweets is at http://tweetburner.com/
Although I created my own background for Twitter, you can try http://www.twitbacks.com/
Analyse your Tweets, how influential you are, at http://www.twitalyzer.com
Twitter uses 140 characters maximum for messages, so use http://tinyurl.com/to shorten your URLs. Tweetdeck does this automatically.
Twitter search engine, one of many out there - http://twittertroll.com/