I am surprised no one has mentioned RISUS yet. I've never played it, but Rel (and Ethernaut? Can't recall if that's correct) both swear by it as an incredibly easy game system. Its rulebook is six pages, including pictures.
I was going to post RISUS, but you beat me to it. Damn you Henry! (and yes, Ethernaut is the RISUS pusher...) Here is is: http://www222.pair.com/sjohn/risus.htm All of 6 pages, and will do anything you want. From one shots to campaigns, it's flexible enough to handle any genre, and you can read the whole thing in less than 10 minutes, and understand it too!
I actually read the mailing list for RISUS and I'd say Risus is simple but also subtle. It usually takes a Eureka-style epiphany to fully grok Risus's cliche concept. Usually first-timers see Risus and think they have to add a rule here or there (or a whole bunch a rules) in order to fix Risus. They post their miracle fixes on the mailing list will tell them "or you could just do it by the book". and somehow that phrase works like a zen koan and the pupil attains enlightenment.