Escapecraft

Another minecraft clone, but this one is open source. Still in the infant stages, but there is promise.

http://terasology.org/

They do have a headless option for server, but it's got problems according to one of the developers, but they are hoping soon to have a stable server out.

Excuse my pessimism, but I really don't see much value in this game. All it is right now is just a early-in-development MC clone, with not nearly as many features as the game it's based off of has.

Not to mention, I crashed after about 2 minutes of playing when I right clicked a block with what I think are scissors? And I fell into lava before that, and didn't take damage? Yeah, definite nope from me.