Snitter, an Adobe AIR twitter client
Yes it takes a download of the Adobe AIR runtime, but what is 9mb's really? Nothing nowadays. Sure it has its good and bad, but give it a shot if you haven't already. DownloadSquad covered it here. Snitter started out as a little project by Jonathan Snook to test out Adobe's AIR. And why did he pick Twitter as a good API to use it with? Bad feelings about the Twitter website that he felt he could upgrade a little. The application works on both Mac's and PC's thanks to Adobe AIR, and is a quick download. It is a compact application that sits on the desktop much like an IM, and can be resized to fit in nicely with what's going on in your space. Tabs make it easy to check out the archive, replies, friends, and followers list, with a clear tweets button and refresh tweets button to clean your space. Messages that are sent directly to you with @, get highlighted which makes them stand out. One drawback is that only the past 20 recent items on your timeline show, which is a little annoying at times especially if you didn't have time to check in one day.
It's a great tool for Tweeting if you don't mind installing the Adobe runtime.