Note: msdn urls often change so if it isn't working search msdn.microsoft.com for rotor.
This is a beta of a FreeBSD build for a Shared Source implementation from MS of Microsoft's .NET Framework or Common Language Infrastructure (ECMA-335 standard) It includes a C# compiler (ECMA-334 standard).
This seems similiar to the Open Source Mono project's implementation which also implements the .NET Framework and has a C# compiler.
I expect that the Mono implementation is conforming to the same open standards. Sun has withdrawn Java as an open standard, I believe.