Mono (software)
Mono is the name of an open source project by Ximian to create a .NET-compatible set of tools, including a C# compiler, a Common Language Runtime, and more, all to run on GNU/Linux or UNIX computer. While Microsoft plans to make a version of .NET available for FreeBSD with Corel, Ximian and others believe their shared source license will accelerate development of a Linux port.
A similar project, called Portable.NET, is part of the dotGNU project.