Crafty is a project mainly written in C# and JAVA, it's free.
Crafty is an optimized .NET port of the minecraft server.
Crafty is a java => c# conversion of the original minecraft server.
Since this project is based on decompiled minecraft server code, I give the full rights to Mojang, so they may use this code as they see fit.
Currently requires Visual Studio 2010 to compile.
Make sure to report back bugs!