Windows only: Fans of TweetDeck with a preference for open source will love MahTweets, a plugin-based Twitter client that also integrates with Facebook, Flickr, Twitpic and more, and offers all kinds of handy customisations.
The first thing you do upon opening MahTweets is choose what plugins you want to install. Right now it's only got a few, but they give you the opportunity to choose what networks you want integrated (including Facebook, Twitter and RSS) and where you want to upload pictures (Facebook, Flickr, or services like TwitPic or yFrog). It then navigates you through the rest of the preferences, in which you can enable notifications, automatic translation, a screensaver based on your networks, and tweak a bunch of performance-based options.
After you're done with the initial setup, you'll see the client itself. It starts off with only one column (shown above), but you can add as many as you want, with all sorts of different kinds of filters in each to keep your groups organized (like TweetDeck). In addition to the basic features for updating your status, sending direct messages, and re-tweeting, you can also take webcam snapshots, upload pictures to Facebook or Flickr, and view links to pictures inline—no more clicking on links to open images in your browser.
MahTweets is a free download, Windows only.