I'm just so addicted to it right now.
Minecraft is a downloadable game for almost all computer systems (PC/Windows, Mac, Linux/Ubuntu, etc).
It's pretty fun; you get to create absolutely anything you want in the game.
There's fighting in it too, but only if you want it because you can change the difficulty at any time.
The game's still in development but it's cheaper to buy it right now than when it's finished. Plus you get the updates almost immediately when new versions come out.
There's all sorts of different servers and maps for you to try out. Plus many texture packages (the way you view the game) and mods (extra little things to enhance experience in-game) to mess around with.
It may seem boring.. But you can get really creative; it's really based on trial-and-error to make things.
And if you don't want to buy the full game, I'm pretty sure you can play offline or in-browser, which is pretty free.
I'm not asking you to buy it. You can probably watch youtube videos about Minecraft. Famous one that I know about is the BlueXephos channel. They're pretty hilarious. :)