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: