Wild Terra Online [Steam, Official Site] is a player-driven MMO with hunting, crafting, building creation and tons more and it's now available on Steam in Early Access.
I've followed the development for a quite a while and it did seem promising a while back. I've tested it a few times before the Steam release and I do like what I see, so I will continue to keep an eye on it as it becomes more complete.
I think it needs work on some sort of introduction, as just dumping you into the world doesn't really feel great. That's a problem with a few of these new MMO games that have been popping up, with a lot of focus on the sandbox but not on teaching people what they can actually do.
I do like the fact that all buildings will be player-created, as it could make a rather interesting world.
It seems they also allow you to make your own server, so that's pretty cool.