Called the "World Builder", it'll allow you to "create, edit, and play" on your own maps, which you've built from the ground up. Terrain, objects, nodes, weather, the works. The World Builder will ship with a demo map, along with documentation to help get you started.
While you're stuck with the four basic terrain types from the game - jungle, city, desert and Tyranid gloop - this is free. And by the looks of it, pretty damn extensive.