Mono

If you have ever wished that writing applications for Linux was a little easier, then your dreams just came true.
Recently Novell have released version 1 of their .NET framework titled "Mono". Which means "Monkey" in Spanish.

Mono V1 includes a C# compiler, an ECMA compatible runtime engine and a whole host of class libraries. These libraries include ADO.NET, ASP.NET and Gtk#. Gtk# is a set of .NET bindings for the gtk+ toolkit allowing you to build fully native Gnome applications, and also includes support for UIs built with the Glade toolkit.
The release also includes an IDE, debugger and help library.

I downloaded and installed the Mono v1 packages with no problems at all using Fedora core 2. 

A nice point to note is that Mono includes support for Windows.Forms via Wine, so if you want to write applications in windows and compile them in Linux you can do this without any issues.
However lets face it, with support for Gnome you'd have to be mad to stick with Windows!

www.Mono-Project.com