OfficeSIP Softphone and Messenger are two enterprise VoIP SIP clients written in C# in .NET Framework. The SIP clients make use of Microsoft UCC API SDK, ensuring the highest quality of audio and video. Compatible with Office Communications Server.
See also open source, cross-platform:
1) simple messenger Brief Msg at http://briefmsg.org
2) MUVConf is a multi-user video conferencing, see demo video http://youtu.be/YrBU-Aqtvrk, download https://code.google.com/p/muvconf/downloads/list
A c# implementation of the achat protocol. It is a seperate library so you can add your own interface. there are also two pre-made implementations. one basic (and incomplete) GUI version and a just as useful console version
Aeru IRC is IRC Client for the .NET Framework. It made with C# programming language and features a full Graphical Interface. Aeru IRC uses XML to store servers and channels and app settings. The GUI uses the power of .NET.