I have begun playing the best game ever. EVER. Second Life is an immersive universe, in which you can create anything, and make that anything do anything. By manipulating primitive 3D shaped (cubes, spheres, etc), and then writing a script for it with the built-in scripting language, you can make anything from airplanes to slot machines, from beds to skydiving units (parachutes included).
I absolutely love this game. You can upload video, audio, image, and many other file formats, and then apply them to things you create. An author who plays the game made his book available, for free, before it was released to the public, to anyone in the game (and it's still availabe for free).
Your character, or avatar, can be anything you can think of. Yesterday I saw both Puss in Boots and Papa Smurf. After which I built a see-through house and flew it about the world.
There are many user-created games within the games, including first-person shooters and RPGs.
Seeing that the main drive in the game is to create and program things, this game appeals mainly to those who have an interest in 3D design and programming (meaning, me, for one).
This game is wonderful.
Second Life.