I have played many online games so I figure it was worth making a list of the ones you should checkout.

AMERICA'S ARMY: I have not spent much time playing this yet but I have heard many good things about this game.

Combat Arms: A free Fps with good graphics by Nexon. The game is free with a few things cosmetic for real money, It is worth checking out but has a good amount of hackers.

EVE Online: A pretty cool loking space MMO just got the trial a few days ago but havn't had much time to play. Heard plenty of good from it.

Imvu: A nice looking 3d chat system the link is a invite to the game if you want to join.

Maple Story: A stylish sidescrolling mmo. Pretty cool but it's kinda a grind fest.

Runescape: We have all played it and have had different opinions on it. I had alot of fun playing Runescape and it is a game that everyone should check out.

World of Warcraft: The soul sucking demon from the abyss. I have started playing on the Eonar server, deffinetly worth downloading the trial but can't blame you if your scared.