The Roslyn CTP is now available for download from Microsoft. It requires Visual Studio 2010 SP1 to be installed - the Express Edition is not supported.
This is a project to make the C# and VB compilers self-hosting. Currently they are in written in unmanaged C++.
It will also open up the compiler API to developers and facilitate a number of interesting developments such as an interactive scripting window for C# (similar to F# interactive). Check it out here: