The Description system is the system powering the Neverwhere fan RPG and recent release ImagiNation.
The Description System uses descriptive paragraphs to define powers, challenges, monsters, characters and everything else in the game world and minimises the mathematics involved to a single dice roll and some simple addition.
A powerful system and a good introduction to roleplaying, TDS would work well for off-the-cuff games and quick conversions of favourite settings as well as for more developed games.
This system is completely open. Do what thou wilt.