This is a cross platform, open source .NET development framework implementation. Sponsored by Novell, which can run the binaries produced by Visual Studio without recompilation.
In the nutshell, Mono is the UNIX version of the Microsoft .NET development platform enabling UNIX developers to build and deploy cross-platform .NET Applications. This is possible because Microsoft has submitted the Common Language Infrastructure (CLI) standard to ECMA and then to ISO.
Well, if you are interested in developing the C#-based components, libraries and frameworks on Linux/Unix, this is the tool. Start download Mono.