Gravitation

 is a buff granted to the player upon consuming a Gravitation Potion. The buff allows the player to reverse the direction of gravity, allowing the player to fall upwards or downwards by pressing the Up and Down keys. This lasts for 3 minutes, but can be cancelled at any time by right-clicking the icon. Note that the player still takes fall damage, even when upside down (for example, you can get killed by hitting the bottom of a floating island).

Gravitation in 1.2
The gravitation effect was changed greatly with the 1.2 update. From 1.2 onward, instead of the player flipping upside-down, the player stays put on the screen, and the view of the world flips upside-down. The down key no longer returns gravity to normal; instead, the up key does this. Relative to the flipped view, other entities appear to fall upward. The minimap is not flipped.