What startups here are focusing on developing on top of .NET?
indexzero: I think that there is a lot of stigma around Microsoft based platforms. But imo C# 4.0 is one of the most innovative languages out there right now. With Mono supporting all of C# 4.0 features (ExpressionTrees and all) I would say, if you're not using it give it another look. My ideal .NET stack doesn't have ".NET" in it at all:C# 4.0
HAML 
LESS
MongoDB