Mono is the name of an open source project by Ximian to create an ECMA Standard compliant (Ecma-334 and Ecma-335), .NET compatible set of tools, including a C# compiler, a Common Language Runtime, and more, all to run on GNU/Linux, UNIX and Windows based computers. While Microsoft made a version of .NET available for FreeBSD called Rotor, Ximian and others believe their shared source license will accelerate development of such a framework.
A similar project, called Portable.NET, is part of the dotGNU project.