At Arctan (Austin Ruby Coders Texas Area Network) last night Eric Mahurin gave an overview of the Grammar project. Eric has created a very slick lex/parser. Interestingly the grammar *is* Ruby. Like Rake, it is not some other syntax you need to learn, but it is just Ruby.
Another intesting tidbit was Eric's use of class recursion.
Class Element < Grammar
This allows classes defined in "Grammar" to use all the class methods.