Some of you have already tried out Darkbattle and NationStates, and myself being a fan of theese games I decided to make a list - that you will add stuff to so that I will get something new, too. I'll start and edit the ones you post.
www.darkbattle.com - DarkBattle
Be the ruler of your own planet - research, build ships and attack ohter planets. Millitary and offensive play.
www.kingdomofloathing.com - Kingdom of Loathing
An adventurer is you. Create your own hero in this parody kind of game - even so it's wery fun and addicting.
www.bootleggers.us - Bootleggers
Be a gangster in the US, stealing cars, organizing crimes etc.
www.nationstates.net - NationStates
Create your own country and decide what laws to accpet and what laws not to.
http://www.neopets.com/ - Neopets
Create your own pet, feed it, train it, play with it and battle with it. Earn money and do different things in the big world of Neopia.
http://www.kingsofchaos.com/ - Kings of Chaos
http://www.darkthrone.com/ - Dark Throne (beta)
http://www.outwar.com/ - Outwar.com
http://www.battleon.com/ - Adventure Quest