MahTweets is Yet Another .NET Twitter Client, that has expanded to include functionality for other social media sites including Facebook, Yammer, Foursquare, Tumblr, and that's just the start.
For Stack Exchange I have created a plugin that brings down question feeds from the sites, and allows you to enter your user ID to retrieve comments and answers to questions you've asked in your timeline.
Its features are in flux at the moment, but we hope to include searching, filtering by tags, etc - it's changing near daily at the moment, but the behavior is very similar to a regular (read-only) twitter client, for Stack Overflow, etc.
It's free - Open Source (although we're running through a version 3 beta period at the moment, so source isn't publicized).
License is MS-PL.
Windows / WPF/.NET 4.0
Paul Jenkins (@aeoth) and Brendan Forster (@shiftkey) are the main minds behind it.
Andrew Tobin (@tobin) produced the plugin.
MahTweets Developers (squatted link)
C# .NET 4.0, WPF, JSON.NET went into this particular plugin, but we use many frameworks, including: SqlLite, TweetSharp, StructureMap, Blacklight Toolkit, WPF Toolkit, etc.
MahTweets also has support to write plugins in Python or Ruby.
Code is closed right now, but if you want to contribute or get access to it feel free to ping @aeoth, @shiftkey or @tobin on twitter and we're happy to discuss.