Actually, you can get a free basic membership in the online game Second Life if you sign up through July 13, because the game is celebrating its second anniversary. It does require a download and a broadband connection, but it runs on Windows or Mac machines. Also, you have to be age 18 or older to join.

"...you will need to enter a credit card to register, but it won't be billed during the free trial unless you select a membership option OTHER than 'Basic.' " [Game On: Games in Libraries]

I wrote about this game in my notes from the Games, Learning, and Society Conference when I heard developer Cory Ondrejka talk about Second Life and User Creation. Fascinating stuff, and you can view it first-hand with the free account.