Terraria is a 2D game that has often (and somewhat unfairly) been called "Minecraft in 2D". Not that the four developers care, since they've just sold 200,000 copies in a little over a week.
Spurred on by word-of-mouth, much of it stemming from the game's supposed similarity to Minecraft, the $US10 game was first released on May 16. At time of posting, it was the top-selling game on Steam, ahead of games like The Witcher 2 and Portal 2.
What's undoubtedly lured people towards the game is the fact it appears to operate much like Minecraft with a pinch of Castlevania thrown in, in that you explore a world and craft things while fighting monsters.
I'd say it's a bit closer to being a 2D Dwarf Fortress myself. Well, a different kind of 2D than it is normally, anyway.