This is the multiplatform CLR implementation. Please see:
http://en.wikipedia.org/wiki/Common_Language_Runtime[
^],
http://en.wikipedia.org/wiki/Mono_%28software%29[
^],
http://www.mono-project.com/Main_Page[
^].
It will allow to run wide class of .NET applications on different OS without recompilation. It really works.
Just install it and see documentation. Using it is easy. Installation on Windows is very usual, never caused any problems, on other platforms is usually easy, too, but it depends. For Linux, it depends on the distro, its installation system.
—SA