SSCLI(Shared Source Common Language Infrastructure)

The Shared Source Common Language Infrastructure (SSCLI), previously codenamed Rotor, is Microsoft's shared source implementation of the CLI, the core of .NET.

Although the SSCLI is not suitable for commercial use due to its license, it does make it possible for programmersto examine the implementation details of many .NET libraries and to create modified CLI versions. Microsoft provides the Shared Source CLI as a reference CLI implementation suitable for educational use.

The Shared Source CLI was pre-configured to run on Windows, FreeBSD (version 4.7 or newer), and Mac OS X 10.2. It is designed such that the only thing that needs to be customized to port the Shared Source CLI to a different platform should be a thin Platform Abstraction Layer(PAL).

The current version of SSCLI is 2.0, which contains most of the classes and features of version 2.0 of the .NET Framework[1]. Unlike the previous version however, it is only supported on Windows XP SP2.

Ref:
http://en.wikipedia.org/wiki/Shared_Source_Common_Language_Infrastructure