Shared Source Common Language Infrastructure (previously codenamed Rotor) is Microsoft's shared source reference implementation of the Common Language Infrastructure (CLI), the core of .NET. Shared source is not real open source (e.g. it's not suitable for commercial use) but it makes it possible for programmers to view the source of many .NET libraries, as well as for hackers to create and use modified CLI versions. Microsoft expects Rotor to be of real value to students learning cross-platform compiler technologies, class system design guidelines, and so on, being the real-world illustration to their lectures.
Rotor comes pre-configured to run on Windows, FreeBSD (version 4.7 or newer), and Mac OS X 10.2. However the only thing that needs to be customized to port Rotor to a different platform is a thin PAL (platform abstraction layer), so basically Rotor is portable to any platform.