Witty

Witty 2.1 is a free and open source Windows twitter client built on .NET and powered by the Windows Presentation Foundation (WPF). The application requires the last .NET framework 3.5.

Pros & Cons

Pros	Cons
Nice interface using WPF	Very slow and buggy! needs more development
Ability to temporary ignore all tweets from specific users!	Right bottom popup notifications for new tweets. They get very annoying fulling your desktop when you there are many notifications at the same time (like Tweetie on mac)
Ability to change the URL shortening service	Doesn’t support multiple accounts
Ability to change the theme
Supports twitter usernames auto-completion and correcting spelling mistakes