Quick-Start rules for RPGs can be a mixed bag. On one hand, they can be a relatively inexpensive introduction to a new game, or a useful demo tool/reward. However, if poorly done, they can also be a tremendous turn-off. Not enough detail, and potential gamers won't have a feel for the game. Too much detail, and the eyes start to glaze over. Below is a list of 5 of the better quick-start RPG efforts out there. (Please note I have not included any free RPGs, as those really don't need a quick-start in the same way as commercial efforts do):
1) Savage Worlds: Savage Worlds doesn't need a lot of help to sell itself, nor does it have a high price entry barrier, but its Test Drive Rules are outstanding nonetheless. Only Smilin' Jack keeps this from an A+ (can't stand that guy!).
2) A Song of Ice & Fire: The jury is still out on Green Ronin's recent offering, but the Quick-Start Rules for this game (available from the product's homepage) give you a pretty good idea of what to expect from it.
3) Shadowrun: I'm not a huge SR fan, but this intro to the 3rd edition of that game is a solid 66-page introduction that does a nice job introducing folks to what it's all about. The 4th edition also has a Quick Start pack, so take your pick!
4) Eldritch: Goodman Games' effort at a new system hasn't really caught fire, but that didn't stop this product from being a nice little overview of the system. Good job on including a free introductory adventure, too.
5) Castles & Crusades: A pretty solid effort. C&C isn't the toughest game to get a handle on, but this freebie does a nice job of explaining the basics of play and the all-important SEIGE Engine. It could use a little assistance as far as layout and ease of reading go, though--two things I think important in a first impression.
This is by no means meant to be a comprehensive list, so feel free to add your own!