TweetDeck is a Twitter client with some useful features. It is an Adobe AIR application which supports groups and has different panes to better categorize tweets.
With tweetdeck you can visualize tweets, replies, direct messaged in different panels. This is done using a local database to store tweets and then categorized in separate columns. It also supports Groups, the much needed feature for twitter. You can select a bunch of twitter followers and add them to a group to better categorize tweets. The other interesting feature of the application is it supports tweets even when twitter is down, these tweets are kept in a queue and sent to twitter when it is up again, currently this function is disabled but it will be available soon.
Overall the app looks good and groups and different column visualization of twitter messages is quite useful. The groups feature is useful if you have large no of followers on twitter and want to separate tweets from close friends. Another twitter client to look is twhirl