Often considered the "2D Minecraft", Terraria has managed to sell over 200,000 copies in nine days thanks to some assistance from, well, Minecraft.
The game has been developed by Re-Logic and is an independent game that plays out like a hybrid of Minecraft, Metroid and Castlevania. This game has created a massive word-of-mouth frenzy within the PC community - and Minecraft helped promote the game.
Released on the 16th of May, after achieving this amazing milestone, it also managed to outsell Portal 2 and The Witcher 2 in that period on the Steam Store.
Re-Logic are also looking into an iOS and the Xbox 360 port of the game. Terraria is avaliable on the Steam Store for $9.99 USD. (http://store.steampowered.com/app/105600/)