There are a lot of online games you could play. Only one comes to my mind at the moment: RuneScape - www.runescape.com If I find some more, I will PM you about them.
As a few people have already mentioned, Gunz online, Wolfenstein: Enemy Territory, as well as Knight Online, Gunbound or even America's Army.
dude LINERIDER ALL THE WAY. google it, seriously man, it's freakin SWEEEEET!! you make a line and you ride it with this cute little dude on a sled. no joke man, it's addicting.