Apocalypse World is what I'd recommend, which gomad mentioned in comments. For my part, I have played and run it, so I feel like I can speak to it with at least a bit of authority.
Apocalypse World has the mixed distinction right now of being the "new hotness"—or at least it did last year during the run-up to its public release and after. It has been called a game-changer, which is high praise. I won't say whether it's true in the usual meaning of the phrase, but I can say that it has changed my personal game profoundly. Whether any of the praise is deserved or is just hype can really only be judged personally once you've played, but it's something to be aware of if you go do more reading about the game online.
The game has three virtues going for it relative to your question:
First, it's a very new game; the most recent expression of a well-respected indie designer's cumulative growth over the course of several successful games. Notably, Apocalypse World has outsold by several times all of Vincent Baker's previous games put together, and games like Dogs in the Vineyard aren't anything to sneeze at.
So, it has the vote of the indie-games-buying market, and you can be sure that there will be lots of online discussion, actual play reports, and accumulated wisdom on how to run the game.
Second, AW is a seamless blend of a complete game and an instruction manual on how to GM—at least, in one specific style. It explicitly says that its way isn't the only way to run Apocalypse World, but it does say that the way to GM that it describes is the surest way for the average GM to run AW in a way that gets the most awesome out of the system.
To that end, it describes this very specific style of GMing in exhaustive but accessible detail, going so far as to create a set of rules for the GM to follow. This manual on "exactly what to do right this minute during your game" is one of the innovations that have got people talking about the game so much online.
One of the principle advantages of the GMing style it lays out is that the GM does not need to have a prepared plot. The game goes so far as to say the GM is not allowed to plot things in advance. This enables part of the core goals for the player experience of Apocalypse World: as a player, you can go anywhere, do anything, and impact the setting any way you'd like to try. You may not succeed, but it will be because of poor planning or failed rolls, never because the GM blocked your actions to preserve the story. This is also great as the GM, because you get to be just as entertained finding out what happens as the players.
The GM "rules" enable this through a set of tools and techniques that are designed to be invisible to the players (and they are!) while being highly structured for ease of GMing the game even for novice GMs. The structure is all about creating bits of setting, NPCs, and other playing pieces the GM can bring to bear to either react to the players, or throw a spontaneous (yet structured) development at them when they're not giving you things to react to.
I could go on, but eventually I'd just be spewing parts of the book. I'll let it be and just finish by saying that this is one of the prime reasons to buy the book, even if it never gets run. As a manual of concrete "do this, then do this" GMing techniques, the book is solid gold.
Third and finally, the game manages to both be rules tailored specifically to the apocalypse world setting and a general rules engine for any kind of setting you might like to create. While the "how to run" rules are given to the GM and make up most of the book, the "how to be a player" rules are (nearly) all on the character sheet (usually a folded one called a "playbook").
Each playbook contains the basic Moves shared by all characters as well as the Moves specific to the class in the playbook. The Moves of a Chopper all involve dealing with your biker gang, enforcing authority, and raiding across the wastes. The Moves of a Hardholder let you detail a stronghold (that exists by player fiat) and exercise authority (or fail to) over your people. The other playbooks contain similar Moves that, together, create much of the setting.
In fact, part of what makes GMing Apocalypse World easy is that the setting is created for you by the players' choices of playbooks—you have a Hardholder, a Chopper, and an Angel? Then your campaign is going to be about the threats and opportunities for the hold, enforcing the "peace" and raiding with the biker gang, and all the blood and guts that the Angel is going to have to mop up and put back inside the people who they value (and why). A game with a Brainer, a Driver, and an Battlebabe, on the other hand, is likely going to be about weird mind-control things (whether the PC's or NPC's) and wrestling with the Psychic Maelstrom, staying always on the move while trying to keep the car fueled up, and getting into (and often winning) nasty fights.
But while the setting is expressed by the rules built into the playbooks, using AW as an engine for a completely different genre and setting is entirely possible just by writing your own playbooks; and playbooks are only slightly more difficult to write than they are for a novice player to pick one up and just start playing the game cold, which is deliberately easy. There is, in fact, an official Apocalypse World subforum devoted to setting hacks, and a D&D-inspired hack called Dungeon World is about to go to the printers (a lite version of which is available for download. Oh yeah, and Vincent Baker is very happy to have people run with his game engine in whatever direction they want, including a commercial venture. Which is a good unofficial fourth virtue to wrap up this already-length answer with.
So in sum, there's a lot Apocalypse World has to offer in mind-expanding rules, both for a game of AW and for GMing and game-designing in general. And if you do actually run it instead of just absorbing the rules innovations, it's a lot of fun to play. I had a great time with my Hardholder character, and I've successfully run it cold-start, with no prep by me other than reading the book, and no previous exposure for the players until I dumped the playbooks on the table. It doesn't hurt that it's downright cheap for a complete game.
