All platforms with Adobe AIR: When you follow enough chatty people on Twitter, some days the noise can overwhelm the signal—that's when you want TweetDeck, a full-featured desktop Twitter client that groups, filters, and searches incoming tweets for you. TweetDeck's killer feature is its ability to separate the people you follow on Twitter into groups—like "Don't Miss," "Co-workers," and "Chatty types." That way while tweets by the prolific folks fly by in one pane, infrequent tweets from people you don't want to miss stay bookmarked on top in another. You can add or remove modules from TweetDeck's multi-paned interface to show you all tweets, tweets from groups you configure, replies to your tweets, direct messages, search results for a keyword, and more.
The second most useful feature TweetDeck offers is its excellent replies capability. Unlike the Twitter web site, TweetDeck doesn't only show you replies that start with @yourusername—it shows you any tweet that contains @yourusername anywhere in the body. Similarly, it can show you a constantly updating search for a particular term, like a search for "Lifehacker." The more modules you add to TweetDeck, the wider it gets. Here's a screenshot. (Click on the image to pop up a full-size version.)
Of course, you can resize TweetDeck to whatever width you want—a single column, if you'd like.
TweetDeck can also display all the tweets from the last 48 hours, often more than the Twitter web interface can (when the "Older" button's disabled, anyway). TweetDeck also has TwitScoop built in, which shows you the most popular keywords appearing on Twitter at the moment, a Twitter status indicator for when the service is down (which is often), and a URL shortener available right below the new tweet entry box. TweetDeck is a free download for all platforms running Adobe AIR.