My friends and I have come across a Minecraft inspired space sim called Starmade; It's currently in alpha, but already has a hefty set of features that lets players build their own fully interactive ships, space stations, and other offensive flying geometry... which you can then share with the entire server through blueprints.

The fun thing about this game is that it will populate the universe with player made ships, as long as they're not being used. Meaning that the older the server gets, the more varied the A.I becomes!

Nyax and I have already found the wonders of using planet cracking monstrosities on high gravity planets.

The only issue I've had with the game so far is that navigation isn't quite done yet; Making it very easy to get lost. But that's just part of the fun, yeah?

You can get the game (for free!) at www.star-made.org.
It also has a subreddit that the developer reads: /r/starmade

After you've downloaded the game, just connect to:
chariot.moon-dog.net:25565

A tutorial will quickly explain the game to you and you should be good to go!

(Don't forget to look through the advanced video options for helpful jazz like draw distance.)

Enjoy!

Update!

So uh, because there's only a handful of blueprints on the server; The A.I is going to try to attack you with UNARMED mining ships.

Hilarity ensued.