Remember Super Mario Galaxy? How Mario could simply walk off the side of what seemed like square blocks, sometimes? This mod adds that to Minecraft, opening up a whole new world of possibilities, for example completely removing the need for stairs or piston elevators.
The first thing this mod adds is Gravity Runes. After you craft them, simply right-click with them to flip your gravity, sending you up towards the sun or moon. Hope you had a roof over your head, otherwise you'll lose your gear to a mysterious space station. Don't worry, though, you can right-click again to bring your feet back down, though hopefully you'll have remembered that fast enough you won't die from fall damage anyway. Use these to make a Gravity Staff, which lets you control the block-based tools in this mod.
Secondly you'll want to make a Gravity Director and gravity helmet. The first item allows you to change the gravity direction of pretty much anything that walks inside them including sand and gravel. Pigs, cows and such can walk up walls, and around ceilings without a problem. The helmet neutralizes your gravity, which is helpful as an emergency tool in case you ever fall off a wall you were walking up.
Torturing villagers is also pretty fun.
This mod also adds a gravity regulator, which controls how high you jump and how fast you fall. If you want to really punish villagers for some reason, simply add a gravity regulator at maximum power, nudge the villagers through the regulator, and they'll take fall damage from a simple 2 block fall.
The third and final thing this mod adds is planetoids. Yes, a miniature planet. Now you don't have to have a floating, one direction base; you can turn your base into an entire planet. There's two ways to escape planetoids; either make a staircase that goes out of the gravity well's range, or put on a gravity helmet and trust you won't die from fall damage.