A new version of the open source Minecrafty game was recently released - Minetest 4.14, and it's surprisingly mature.
It's designed around plugins/mods and subgames, and it doesn't have a height limit like Minecraft, or a depth limit, but if you delve too greadily and too deep, instead of a balrog you may find a Nyan Cat.
(Actually the monsters get pretty nasty the deeper you go)
I've set up my own server at minetest.openblocks.com if anyone is curious, though there are plenty of other servers (@Minetest ), and mine might be limited by the Australian Copper Network (I can't tell — my client has a LAN connection to the server).
There's no authoritative game: the landscape, the recipes, the mobs, the blocks, the items, the game mechanics etc are all determined by which plugins you throw at the server. I've tried to set mine up like Minecraft SMP but without the plugins that are trying to clone Minecraft - not being exactly the same makes it more interesting. Though I did add a Nether plugin, because I wanted to see what their take on the Nether would look like.
PS: If you run the game client without connecting to a server or configuring your own, it won't have mobs - the default plugin list is quite minimal. I think mine actually has too many mobs, and Minetest mobs look like they are just proof of concept at the moment, which might be why it doesn't include them by default - mobs is an area where Minecraft shines.
PPS: The engine may be mature, but there isn't a mature SMP subgame, I've just slapped the components of one together, it's not planned coherantly or balanced or anything.