I've recently got into War Rock (also searchable under Warrock), which is an online FPS (first person shooter) similar to counter-strike but imo better.

http://www.warrock.net/ theres a link for anyone interested, it's free to download from download.com and will run on most pc's that run windows xp

before that my main addiction was Freelancer which is a space flight sim, a damn good one, pretty big game , lots of solar systems, bases, planets and stations to visit (all named after real places... like the "New York System" which houses "Planet Manhattan" and other such places) its easy to get the hang of too... heres a screenshot from a while back.
http://i74.photobucket.com/albums/i2...ls/screen1.jpg
(i was running a crapy laptop at the time so my graphics specs were real low, its usually a very very good looking game)
The best thing about this online game is that different servers run different "mods" which customize that particular server, for example you could have a server running anything from starwars based ships/planets/systems to star trek based ships/planets/systems and also popular is Battlestar Galactica based ships/systems/planets... but you'll find most run an even mix of all three, as well as some original stuff too, lots of servers create their own entire galaxies and ships and stuff, it is very cool.

here is a link for anyone interested http://www.microsoft.com/games/freelancer/
But i hardly ever play that anymore
Wesley Pipes Reviewed by Wesley Pipes on . Addictive Online Games Have you ever started a game thinking you were just going to kill some time and the game becomes so addictive that a year later you are still playing it? Post your online game addiction....... I play this stock game called Alexadex. They give you 10,000 Alexadex dollars and you buy shares in various websites. You start by buying the cheap dollar stocks then you move up to the expensive sites like youtube and UPS. You have to buy low and sell high. Just like the stock market. It is Rating: 5