OpenArena is a 3d game creation engine. It is a rendering system that can be used for more than just games but it is targeted at games. The basic use being create some game media textures models maps and config files for physics and rules.
Alfaws is a customizable, multi-user game engine. Every object, character, & room is programmed to respond to events. Players can customize their own event handlers, thus changing their fighting style or having their character behave as they would.