Spaz is another free Twitter client for desktop worth checking out. It is open-source, and because it runs using Adobe AIR you bet it’s cross-platformed. Spaz performs most of the functions a normal desktop Twitter client would (send/receive tweets, stream public tweets, etc) but extra customization support. It comes with 4 themes/skin to choose from, or you can even define your own using CSS.
Did I mention it’s also ads free? Download Spaz now.