YetAnotherForum.NET (YAF)

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YetAnotherForum.NET (YAF) is The Open Source Discussion Forum for sites running ASP.NET. 100% written in C#, YAF (pronouced like "laugh") is a unique combination of Open Source, Microsoft's .NET platform and an international collaboration of the .NET developer community. Growing and changing daily, the YAF project is lead by Jaben Cargman of Tiny Gecko. YAF runs on any web server that supports ASP.NET v2.0 (and above) and SQL Server. YAF is licensed under GPL.

http://sourceforge.net/projects/yafdotnet/
http://www.yetanotherforum.net/