Maude is a programming language based on rewriting: http://maude.cs.illinois.edu/w/index.php
It's pretty wild; your code doesn't "execute" in the traditional sense; instead patterns match the code and transform it repeatedly until it reaches a "normal form" - one that cannot be rewritten further.
Maude is handy for playing with optimizers, because you very often want to specify behavior in terms of patterns (when you see these functions together, replace them with this...)
