You may or may not have heard of Minecraft which is an incredibly fun (IMO) game where you can basically build what ever you want. Essentially it's like Legos. It's in beta now and can be a little bit buggy, but still it's a lot of fun. Always wanted to build a mansion? An underground fortress? A dragon lair with lava (like in Hz ^^)? You can build it! I haven't built anything particularly big like some folks have... yet. It's fun just to even run around and explore since each map is randomly generated. It has different environments, floating islands, and massive underground tunnels. Check it out if you haven't.
My house. It has a big ol' tree in front so you can't see much. :P
Living room complete with fireplace. ^^ Further back is the kitchen.
Oh, and don't go out at night. That's when the monsters come out! Unless you've got armor and a sword then you'll be okay.
Don't blame me if you forget about everything else to devote time to building your massive structure. >_<