Sky dimension

The Sky Dimension (otherwise known as the Floating Island Dimension) is a work in progress by Notch. It "performs well, [but] looks ugly". It is now a confirmed update, although it won't be implemented in 1.6.

The Sky Dimension was first announced after Notch played Spleef against @Evil_Notch (video is on twitter). It was probably made to be like a "heaven", since the "Evil Mojang" team declared the Nether to be "hell". The sky dimension was not completely confirmed until Notch released an update via Twitter stating: "And because PC Gamer made such a big deal of the sky dimension, I guess I'll have to finish it. Consider it confirmed."

Other info

 * The "Sky Dimension" may have been inspired by the Aether (an unreleased mod) which creates an alternate, heaven-like world. It was inspired by the suggestion thread of GreyAcumen which was the first to come up with the Aether and document it. He was inspired by the Nether, after its hellish appearence.
 * It is currently unknown whether or not the "Sky Dimension" will have any of its own items or mobs, although the pictures at this point seem to suggest that it will not.