Estelle Pienaar has an in-depth review of the Grim Combat System (GCS), which claims that 80 regions have shooter games created with its system. That's quite a lot, and reading Estelle's review, it's easy to see why:
GCS does not only provide weapons and a meter that measures the players health. They provide a whole game construction kit for region owners. The GCS system includes - amongst many other items - all kind of monster spawners; exploding cars, barrels and dumpsters; low lag city building kits; scoreboards; recovery systems; fog machines; breakable glas; quest items and even quest NPCs (non playing characters).
A zombie game called Axis of Evil is built with the GDC system, and while the graphics and the MOBs are pretty primitive, it looks like a genuinely fun game for that genre: