I guess this could also go in the "Google's new thing" thread.. anyway:
Allows you to make edits to google maps. They've had a "report" feature which I've used twice to let them know that construction on a particular road has finished. However the reports process takes a long time (on the order of weeks / months). These Map Maker edits are reviewed in real time by other users (and presumably eventually reach somebody with some kind of authority at google).
I'm looking forward to a much more responsive map of the world (especially since I use android navigation) :-D