I just bought Rust off Steam, haven't had the chance to play it yet, though I can see a few Steam friends have it already.
http://en.wikipedia.org/wiki/Rust_(video_game)
http://playrust.com/about/
http://store.steampowered.com/agecheck/app/252490/
It's a game inspired by the likes of Minecraft and DayZ, created by the team behind Garry's Mod. You're on a big island and the goal is to survive: you can craft weapons, chop trees, hunt animals, eventually somehow get hold of firearms, build bases (check their website, some pretty huge forts there ).
It's basically a more FPS-y version of Minecraft, or DayZ without zombies and with building. It's Alpha, so very little of what it will be currently exists. Anyone remember MC Alpha? It was pretty basic. This game shows great promise, though.