Unreal Tournament, up to the 2004 version if I recall. It's a nice alternative to Q3:A for those using netbooks that don't have OpenGL support - trust me, I've been there. Try and avoid your netbook's keyboard and trackpad, though.
That's pretty debatable, Gmod can get pretty intensive when there are a lot of physics objects going on. In fact, most Source games are a little demanding for straight up potatoes, with CS:GO being a notable example of shaders and Portal 2 being CPU intensive at points due to the physics going on.