http://www.blockaderunnergame.com/
Minecraft in space. But not really minecraft, except it has blocks.
Some of the promised features:
* You design your spaceships in an editor, then get them built and repair them in a shipyard
* Multiplayer
* Physics, it matters where you place the engines
* Sensors, radar etc
* Prefabs; non-block placeable items. Everything from guns to doors to plants and displays. You can also make them yourself.
* Planets, asteroids
* Ships inside ships, and ships that can separate into more ships
You can also download an early invdev for free right now, build a ship and fly and shoot it offline.