Lately I've been playing a lot of Minecraft.
If you haven't heard of it before, it's definately worth checking out. Essentially, it's legos for adults (well, debatably autistic adults).
What's the purpose of the game? Nothing. You create your own purpose.
Go out and find some materials and build something.
You want a nice little log cabin on the shore of a calm ocean? Better start chopping wood. Hope you made an axe, otherwise it might be tedius.
Want a huge stone castle? Hit the mines and gather up stone - because you're going to need a lot of it.
Pretty much anything you can think of could be built - if you put your mind to it. Unfortunately, that's what the biggest downfall of Minecraft is - if you don't really want to build anything awesome, the game isn't really going to provide much else. You can check it out at http://minecraft.net. There is reportedly a demo available (only classic version), but you're probably going to want to end up getting into the Beta for roughly $15 USD.
Why am I freaking out about it?