When Dawn of War II's next major update - "There Is Only War" - is released sometime during the summer, it'll bring with it a much-needed feature for the game: a map editor.
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.