Let's see...
Star Wars Saga Edition RPG.
Star Wars RPG Second Edition (West End Games).
Paranoia XP (I am cleared, thank you friend Computer!).
D&D Rules Cyclopedia (1991).
Shadowrun 3rd edition (mostly because I haven't gotten around to looking at 4th yet so I dont' know if that'll be better or not).
I think those are all the single-book rules systems that I could pick up and either play in or run fairly quickly.
HERO System, 5th edition and GURPS don't qualify for me, because they really do require a character builder program to keep the math straight; I couldn't just pick either one up, create a character, and run with it.
Tekumel and Talislanta are a bit unwieldly, not to mention Tekumel has a very different mindset necessary to run and play in.
I'd have nominated Amber too, but it kind of requires the Courts of Chaos book and is a very difficult game for me to run.