Polymorph

From the introduction:

Polymorph is a minimalist, generic role-playing system, designed to allow players to describe any character they wish and give them plenty of options which will determine their chances of success. Narrators should find that characters and creatures are easy and intuitive to flesh out and that running even large and messy combat scenes is easy and intuitive. Both players and narrators have a meta-game resource which allows them a limited ability to push the plot one way or another.