My group has been on a long string of GM-less or GM-light games, since having kids really cut down on my prep time. Before that, we were playing straight D&D 3.5, with the occasional game of Savage Worlds thrown in.
Here's a sampling of the stuff we've tried:
Microscope: a GM-less game that is all about creating a world and the major events & characters throughout its history. You zoom around the timeline in a pretty crazy sense, painting the broad strokes before diving into individual scenes to see how that event played out in history. My group liked it, but it's a bit long for casual play. My feeling is that it would be far better to play with a bunch of "writerly" folks. Perhaps a whole group of former GMs.
InSpectres: a game that requires little-to-no prep time, and is very easy on the GM. The basic premise is that your group has opened up a new franchise of InSpectres -- a national ghost-busting chain. "Fighting the forces of darkness so you don't have to!" and "Because it's not just any vampiric infestation -- it's your vampiric infestation" should give you a pretty good feel for the flavor of the game. There are four stats, no character improvement, no HP, and the players get to resolve their own actions and add to the narrative when they roll really well. It also features reality tv-esque "confessionals", which allow the players to further tweak the flow of the game as needed.
As the GM, you're pretty much in charge of keeping the pace, calling for rolls and making sure the players don't wander too much. You can't really control the story, because the players may interpret events entirely differently than how you intended, but it's worked out really well for my group.
Fiasco: Our current favorite, and go-to game. Fiasco is a GM-less game in which you play people with big ambitions, pressing needs, and poor impulse control. The whole point is to tell a story about crazy plans going wrong, capers blowing up in the middle, and relationships unraveling. Your character rarely survives unscathed, and character death is a real possibility.
The game is divided into four parts: Act 1 (everyone gets two scenes centered around their character), The Tilt (where things go horribly awry), Act 2 (where everyone gets another two scenes, and the whole house of cards begins to fall down), and the Aftermath (where people get a few sentences to resolve what happened, according to the number of dice they collected during the game).
Fiasco games for 4 people last between 2 and 3 hours, and often feel like playing a movie. It's just a fantastic amount of fun, and I can't recommend it highly enough.