Planet Explorers has one of my favourite premises for a game, which is: you've crashed-landed on an alien planet, now go off and explore and survive. The way you do this is by engaging in a game that is somewhere half way between Minecraft and an MMO. (You start off getting a mission from someone with an ! above their head, and that mission is to hit a tree so that you can gather materials to build stuff.) It's in alpha right now, but it's on its way to being a big builder's sandbox, complete with a voxel landscape that you can deform, or at least run around in digging holes with a shovel, as I did for the first half hour. It's a world filled with alien plants, alien beasts, and towering robot things. The game is in early alpha right now, so it's very much a work in progress, but it's a (currently) free game that already does do what it's says on the tin. There's a trailer below showing off some of the delights of the latest build.
You can play it for free, just here.