If you're in the market for a lightweight, open source microblogging client for the desktop, have a look at Spaz. It's a slick cross-platform app that uses Adobe Air to update your status on Twitter, Identi.ca, and Laconi.ca.
Spaz supports multiple themes, event sounds, and user-defined CSS overrides. You can delete updates you've already sent, Favorite your friends' Tweets, and send direct messages or replies to other users. Spaz keeps track of who you follow as well as who follows you, and auto-refreshes to keep you on top of all incoming direct or timeline messages.
Spaz has a bunch of customization options and some and wonderfully geeky themes like Dumb Terminal.
The drop down menu gives you quick access to the most-used commands, and intuitive buttons let you switch easily between your friends' timeline, updates you've sent, search, and direct messages you've received.
Of course, status updates are more than just text these days, so Spaz also includes support for image posting services like Twitpic and yfrog, and several URL shortening services like bitly, snipr, and isgd.
The project's team recently released a preview version of Spaz Mobile for the Palm Pre, with support for more mobile devices coming soon. Future plans for the desktop app include a complete rewrite from the ground up, and an eventual move away from Adobe Air to Appcelerator'sTitanium platform. Don't let the team's long-term plans stop you from downloading Spaz now, however, because it's already a great little app in its own right.