Really, only the one - WoW. Started that in August 2005 and still playing on and off. As most of you know, I have never and will never be hardcore at it - by that I mean, I never have time to raid in an evening (my woman doesn't really like that!) therefore I am stuck with soloing and grabbing some instances when I can. Never really been fussed though.
Started with my Paladin, Thugwar, hes sat in Outland somewhere still at level 66 - (Pala class sucks big time), moved on to my mage Smallbloke and then finally my BELF Huntard, Foirroo (Hunters - easy mode completely - target, auto shot, go and make a brew, come back and loot, rinse and repeat)
I have tried Star Wars Galaxies - never got on with it - although, technically that was my first real MMO.
City of Heroes - this seemed to be a good effort for an MMO and if hadn't been for WoW it could have been quite good, but it actually is quite dated now and the quests seem very samey.
LOTRO - Never go on with it. The issue is with, ther are stuck with LOTRO lore and therefore are limited to the realms of that.
EVE - this is the sort of game I should love and maybe should revisit sometime.... Do they still do a free trial?
Got high hopes for the new Star Wars game at the moment, I really do hope they make it something special but a part of me suspects that it won't compete with WoW. And if it doesn't I think I might as well quit with MMOs completely.