I love geofiction—creating imaginary worlds through maps—and OpenGeofiction is something I’ve wanted to see for a long time: a collaborative map of an imaginary world that is built with OpenStreetMap’s editing tools. The world is divided into territories, some of which any member can edit, others that are assigned to individual members (after a waiting period). More info here.
I’ve been playing with it and am already nervous about the amount of time I can see myself losing to this. (Though one wrinkle is having no real-world referents to determine scale: without GPS traces or aerial imagery, figuring out how big a house, or a cloverleaf loop, should be is going to be tricky.)