OMeta is less than 2 years old and is a language for creating other languages. In my opinion, it's more expressive than languages like Microsoft's upcoming MGrammar.
The coolest idea it has is that you can create languages in an object-oriented fashion (e.g. you could create a mashup language of Ruby and C# grammars).
It's also nice that the syntax looks almost identical to BNF and thus quite readable.
I wrote more about it here.
