COmega
COmega is a research language (from MSR, where else) which gives really powerful extensions to C#.
To summarize, there are two extensions : X# (Xen) and Polyphonic C#. Check out COmega Home for more on what these really are.
You can download the COmega compiler preview from the MSR downloads page. This includes VS.NET templates, command line tools, documentation and lots of samples.
I was blown away by the raw power Comega provides as a programming language. So much so, that I am really interested in joining a COmega SIG, if there is one, or probably start one, perhaps !!!
I have not yet really delved into all aspects of the language (especially Polyphonic C#), but will do it and will definitely come back with more posts on the same.