TweetDeck is a pretty neat program. The selling point is the ability to split your main Twitter feed into subfeeds that you can then organize into different groups. You can have Twitter be as narrow as a single column while you scroll back and forth in it, or you can make it full-screen and have your monitor filled with nothing but tweeting goodness.
There's a lot of features as well. If you go over the 140-character limit, you can hit a "TweetShrink" button that will turn some of the words to LOL-speak in order to fit the limit. You can upload pictures -- clicking on the TwitPic button will upload the image file to TwitPic and give you the URL for your feed -- and you can shorten URLs via your choice of service by clicking the "Shorten" button.
Another very cool feature is the ability to translate your tweets into another language. The translation is pretty accurate -- even for more difficult languages such as the Japanese I tested with the application. The program itself is highly customizable, and you can tweak the colors and fonts to whatever you like. You can also set different update times for various feeds: for example, you can have your main friends update every minute, but your private messages every five minutes.
TweetDeck is extremely polished for a beta client and I've yet to get the error messages that plagued me with Twitterrific. If you're searching for a free desktop client for Twitter, it's hard to beat this one.
TweetDeck is in public beta and requires the Adobe AIR runtime, which is a Universal binary and needs OS X 10.4.11 or higher. AIR will be installed during the TweetDeck installation process if you do not already have it.