wolfenstein enemy territories is a great game for free, same with americas army. the fact that on ET there is mods and tweaks and everything make it bad ass

i play alot of games online, and even more offline
wolfenstein enemy territories
Age of empires 3
age of mythology
americas army
world of warcraft
diablo 2
diablo 1
call of duty
battlefield vietnam
soldat
halo

thats pretty much all the online ones