Minecraft

I've been playing alot of Minecraft lately.  It's creator has described the game as a mix between Roller Coaster Tycoon and Dwarf Fortress.  Dwarf Fortress having an immense learning curve but an super impressive dungeon crawler, and the iso-metric ease of approach of a Roller Coaster Tycoon type game.  The game is still in Alpha development, but has already sold an amazing 600,000 copies.  With my love of rogue-like games, this is a fun dungeon crawler, and it has cool physics and mechanics.  And it's still in Alpha development, so it should eventually be a barn-storming smash (if it isn't already).
Perhaps a better name would be mine-crack, because the mechanics of the game allow for really interesting virtual worlds, and can become an immersive time sink.

Here is a random video showing some of the electrical and mechanical complexity you can create: