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
My-WIM is a new instant messenger service, which provides chat, white board, voice and video chat, games, file share, file transfer and many more features. Active development started!
Social Networking Features include multiplayer 3D Billiards, Personal Web Server, Web Cam Sever with motion detection, Web Cam Client, VOIP Voice Phone, P2P File Sharing, Instant Message, Chat Rooms and more. 3rd Party Plugins Welcome :)