I don't know how easy it is to develop these, but Dota 2 has a pretty big custom game scene that is somewhat supported by Valve (they explicitly support it, hos

I don't know how easy it is to develop these, but Dota 2 has a pretty big custom game scene that is somewhat supported by Valve (they explicitly support it, host a lot of the custom game servers, and have the custom game browser built into the client... but they don't always address problems timeously).