It may not be the first GM-free RPG I've tried (that'd Baron Munchaussen), or the one that won me over to the concept (that's Polaris) or the one I think has had the biggest global impact (Fiasco), but I have gotten some serious mileage out of Microscope.
Microscope definitely plays with some of intrinsic concepts of role playing games. You don't play specific characters, although you might still play someone in any given scene. Instead, you are basically omnipotent gods who ignore space and time in order to create history.
In essence, Microscope is about creating a timeline. At the start of the game, everyone agrees on a beginning point and an end point, along with some guidelines (like magic is real or no Nazis, for example, both of which have shown up in games I've played). Then, you take turns adding stuff to the timeline, ranging from periods to very focused scenes.
Here's what makes Microscope really neat. After you agree on a theme and some guidelines, there is no further conversation. This is not design by committee. When it's your turn, no consulting with anyone else. You are Johnny-on-the-Spot and its all up to you.
Which leads to a lot more surprises and creativity.
When you play face-to-face, you actually role play out scenes, the most discrete and focused element in the game. When we've played by email, the person whose turn it has just written out a scene, like a little short story.
Oh, and in Microscope, you are not limited to just going forward in time. You can jump back and forth along the time line like Doctor Who on six expressos. (If caffeine has any effect on Gallifreyans) I've seen games where you go from the space age to the Stone Age in two turns.
Microscope plays plays with time and collaborative story telling and world building in ways I haven't seen before. It's very accessible but it also works very well. I have had a lot of fun with it.
Seriously, Microscope lets you play with the vastness of time and space. It's good stuff.