Ive recently started playing this great free first person shooter called nexuiz, its got a mac, pc and linux version, its sort of like unreal tournament, check it out : http://www.alientrap.org/nexuiz/
Also, check out the spidflisk or whatever they were called servers, i just tested the dm server of it today and it had awesome new maps, they dont repeat old maps that much i usually play ctf beckstein but im getting bored of the same maps.