UPDATE #2: re-worked this app as Witty.
UPDATE: I added the source to CodePlex. http://www.codeplex.com/WPFTwitter
Here's a fun little Twitter WPF client application that I've been creating in my spare time. When I get a chance I'm going to blog more about it. In the meantime take a sneak peak. It's still very beta. Let me know if you run into any issues by posting comments to this blog post. Requires .NET 3.0 Framework. Also I've only tested this on Windows Vista.
ClickOnce Installation Url (Works out of the Box with IE. For Firefox, use the FFClickOnce plugin)
http://vertigo.com/downloads/wpftwitter/publish.htm
If you don't know about Twitter, you can read one of the following articles about the service.
http://www.time.com/time/business/article/0,8599,1603637,00.html
http://www.msnbc.msn.com/id/17888481/site/newsweek/
http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/03/19/BUG31OM9RN18.DTL
Also check out GoogleMaps+Twitter mashup http://twittervision.com/