Terraria came out of nowhere to become one of the best indie games of 2011. It's essentially a cross between Castlevania and Minecraft (Craftlevania?); blending infinite creation and multiplayer world building with exploration, platforming and epic bosses. Personally, I actually prefer Terraria to Mojang's effort because there's more gameplay to it, at least in a traditional sense.
For less than £2, it's an utter bargain.