This is too cool not to point out: a C# compiler written in C#. It looks like it's roughly beta quality, but I can already think of a few uses for it even so. The parser/lexer alone could be quite useful.
The author has put the source code in the public domain, so it's available to use however you like. Looks like he might also create a SourceForge project so the code can be collectively improved upon in a more organized manner.