Been looking for one :/

Not really, no. There's that Fallout 2 MMO but I think that requires you to own Fallout 2.

http://www.apoc-online.net/

It's the only free one out there. It's alright, but it's not the best one out there, if you're really serious you're going to have to buy one. If that's the route you want to go on try Fallen Earth.