Although ultimately a simple game with somewhat limited replay value, this zombie defense game brings something I've never seen before in the "real time strategy" world: no central base. You are continuously building and breaking down your defenses and base-type structure as you move through the map salvaging materials and gathering survivors. The goal here is not expansion but careful, balanced transition. Move too slowly and you'll run out of resources, but move too quickly and you'll overextend yourself and become vulnerable.
A less revolutionary game, but with amazing replay value is Bubble Tanks 1.5. I've played and set aside $50 games before I was halfway finished with this free, online game. Awesome!