I've talked about the mono project before. It is an open source project that is converting the .NET Framework to non-Windows platforms including Linux (Red Hat & SuSE) and the Mac OS X. There is also a Windows installer for using Mono on Windows.
A C# compiler was created a long time ago, and it works great. All of the C# code from Sams Teach Yourself the C# Language in 21 Days was ran through this compiler. The C# language was fully supported. What has been lacking are classes from the .NET Framework. Now with a first beta released, some of the classes are available.
Note that this is the first of two betas. It does not support all the classes yet. If you are interested in .NET on non-Windows platforms, this is worth a look!
# # #