The Star Miner Mod adds planets and a space dimension!
In 1.6.4 Star Miner Mod Showcase:
This Minecraft Mod adds in planets with their own gravitational pull. You can actually create your own core which can be used to create gravity on your own structure or planet. This enables you to walk upside-down and in all directions. You can even plant trees and crops in different directions. The most astounding part of this mod is the space dimension. If you reach a certain height you will be teleported to a space dimension which is filled with planets and some of which have rainbow colored rings around them.
Get your Minecraft Mod here: