https://www.youtube.com/watch?v=86kXJ73q3q0
If you havent seen this yet and you're bored, watch then play its pretty cool. Basically some devs got sick of waiting for notch's adventure update and made their own in the sky. There are dungeons, mountable flying shit, and a whole new tier of items.
And if you fall to the bottom of the new realm you actually fall from the sky of your normal save to the ground, and either die or activate a parachute. SO COOL.