Particle_Man
Explorer
I'll add "Don't Rest Your Head" to the mix.
I think Risus hits a sweet spot for simple and generic, but still viable and useful.I find Risus to be both simple and fairly useful. It's free too. Very easy to make up stuff on the fly for it.
YOGC Miscellanea
Scroll down to the Crappiest RPG on Earth and UCrap.
Not sure if it counts, but it's definitely simple.
I'm not keen on the probability spreads in Risus, but apparently a lot of folks make 'em work.