Be careful, the link for that free Terraria download might contain malware.
Released less than a month ago, on May 16th 2011, Terraria is a role playing game that is action and adventure oriented.
Similar to Minecraft, Terraria is what in my day would have been called your typical town/dungeon crawl. You explore and adventure, perform tasks, etc.
Maps, monsters unique to certain maps, and non-player characters that you might be able to encourage to come dwell in buildings or structures that the player owns or has built.
A 2-D game, it reminds me a lot of Below the Root of Commodore 64 fame.
With the popularity of the game rising, so is the unfortunate problem of people using the name to disseminate malware and viruses by offering a free downoad. But if you download, instead of the game you get a very large headache.
A rash of links in YouTube videos, Twitter, Facebook, and other social media sites have appeared.
These are not valid links to download the game for free. You have been warned.
Go get a legitimate copy of the game and play in safety.