There are endless Doom source-ports, quite a few Quake source-ports, ioquake3 is still alive and kicking and spawned Open Arena. I don't think it's hugely mains

There are endless Doom source-ports, quite a few Quake source-ports, ioquake3 is still alive and kicking and spawned Open Arena. I don't think it's hugely mainstream but the Doom and Quake engines have been modded endlessly.