Boo
seems like a very cool language. Is it 100% C# compatible? I mean: can I use any C# DLL/class? Could I make use of the XNA framework?
As far as I know, Boo has an implementation on top of the .NET CLR - which implies that it should be able to both consume, and be consumed by C# code. The syntax may not always be pretty when consuming Boo from C# - but the opposite should be quite elegant, given Boo's syntax.
Also, all of the classes in the .NET BCL should be available to you in Boo.