If you haven't heard of Second Life, run for your (first) life RIGHT NOW. I currently refer to it as my crack pipe. The concept is a 3D role playing game like Everquest, only the plot, the landscape, the buildings, and pretty much everything is made by the users. Just to kick you in the teeth, the download and the first week are both free.
I am actually glad I don't have any money right now, cuz if I did I would start paying for this game and subsequently cease to exist. It's EXTRA bad for anyone with even rudimentary 3D, Graphics, or programming knowledge, cuz those people will find the world customization ridiculously natural.
This of course leads to my theory of the addictiveness of online roll playing games, but I'll save that for another day.