Novell Mono creates an ECMA standard compliant .NET compatible set of tools, including among others a C# compiler and a Common Language Runtime (CLR). It also contains additional tools and libraries not found in Microsoft .NET. (per Wikipedia Mono and the Mono website)
Mono runs on Mac, Linux and Windows, as well as other platforms. Releases of Cobra are tested on Mono and a UNIX-friendly package is provided for Mono users on Mac, Linux, BSD, Solaris, etc.
Major portions of Cobra have been developed on Mono (as well as on .NET).
Main site is at mono-project.
Community site is at gotmono.
Previews of upcoming versions of Mono are often available for download.
On Mono, "msbuild" is called "xbuild". See MsxBuild.