The other day, I was going through a couple of Twitter libraries and apps made in C# and here are some that caught my attention:
tweetsharp - A feature-complete library for the Twitter API. Use this library in your client applications to enable a simple, discoverable fluent interface for building Twitter queries, as well as additional features like asynchronous processing, server-side caching, url shortening, and rate throttling.
Yedda Twitter C# Library - The Yedda Twitter C# Library / or rather a wrapper provides easy access to the Twitter API.
Witty - Witty is a free, open source Twitter client for Windows Vista and XP powered by the Windows Presentation Foundation (WPF).
Csharptwit - The Csharptwitt is an Implementation of Twitter Api in C#.
twitterizer - This net project provides an easy to use, object-oriented interface to twitter's online API
You can check some more C# libraries over here