I've tried alot of Twitter clients and this is the only one to offer me a basic white, simple UI. It's perfect for the top corner of your desktop, you can set the transparency etc. They had some problems with Adobe Air early on, but it's all resolved now.

I liked what I read about this program, so I downloaded it and almost installed it. Then, I found out that it uses Adobe AIR. I really do like the idea of Adobe AIR, but I don't like to encourage those big companies. I want something that is FLOSS (Free, Libre and Open-Source Software).