Mantic Games Deadzone... it's not open source but they don't require you to use official models, the box comes with all the army lists, they don't change the ru

Mantic Games Deadzone... it's not open source but they don't require you to use official models, the box comes with all the army lists, they don't change the rules every 10 minutes and the factions are all suspiciously... familiar (without breaking copyright) and you can just drop in most of your existing 40k miniatures