Minecraft, Garry's Mod and rFactor are the best community built games I can think of. Garrysmod was a basic sandbox and the lua scripting community and modellers have kept it going for five years with NO content from the creator except source engine updates. rFactor can be downloaded with one car and track and I could FILL UP a hard drive with mods and tracks and plugins all community made. And minecraft. F__KING MINECRAFT.. Without user content for land purchasing mods, sound mods, texture mods and everything else nobody would play it.
So what does Outerra offer? Anything you want to add to it.