There are a ton of MMPORGs (Massively Multi-Player Online Roleplaying Games) that you can pay for. There is the most excellent
World of Warcraft, the old and dire
Neocron, the "once-good-but-no-longer"
Star Wars Galaxies, not to mention
Everquest et al. TONS.
But if you would rather throw caution to the wind, and even save a few quid you could play a
free MMO.
For instance, there is
Dofus - which you need to actually go see ;) - or something like
Guild Wars which is making itself quite the known one across the glove for being not only free, but pretty and giving you a "WoW" like experience... sort of. :)
To that end, go visit
OnRPG.com for a list, and forums for, free MMOs (to save you manually searching!).
I'll chuck that link somewhere on the left for.. reference or something. Aren't I good to you peeps?