Terraria is a runaway success of Mojang's Minecraft and it builds upon the core concepts of Infiniminer. This 2D platformer appeared on Steam and sold over 50,000 copies on it's first day!
Terraria is a unique experience and has an identity of it's own, in Terraria you can deform and reshape randomly-generated terrian. You start off with a basic set of tools and upgrade to higher tiers by collecting resources from the environment or slaying bosses.
In Terraria you're free to do as you please, but like Minecraft you'd better seek shelter during the night. During the night phase of the game hordes of evil Zombies and NPC's will lay into you. If you're caught off guard this can mean for a rather quick death. In order to survive in Terraria you must build up a base; starting from the most basic of structures but quickly evolving into a huge fortress complete with NPC's.
Terraria is bargain at only 6 quid and features multiplayer for up to 5 players. I can't really sum up this game in words so here's the trailer!