Mono's C# compiler-as-service -- now works in .net too!
IronRuby+Rails+Rack+IIS7 -- between this stack and the same for JRuby in Tomcat. cross platform web-apps have become fun!
No Bugs (e-book).
Axum (Concurrency focussed language on .net) for VS2010
Snap! -- Aspect-oriented support library for .net
Iron* console for VS2010.