EXP bonus abilities

Level Up is a support ability, mostly found on pieces of equipment, namely accessories, that usually doubles the amount of EXP that a character earns from battles. It is an ability that is usually gotten around midway to late in the game.

Final Fantasy VI
Level Up is an ability found in EXP. Egg, which can be equipped by everyone. It is found in Darill's Tomb, in a hidden room after completing a puzzle. It can also be obtained from the Colosseum by betting a Tintabar, or morphed from Harpy, Harpiai, and Aquila.

Final Fantasy VII
Level Up is an ability found on the EXP Plus materia. At Level 1, it increases EXP earned by x1.5, and at Level 2, by double. The EXP Plus materia can only be obtained after Meteorfall at the Gold Saucer, in the Wonder Square, for 1000 GP.

Final Fantasy IX
Level Up is an actual ability that must be learned, and can be learned by everyone. It is learned through Iron Helm, Egoist's Armlet, Extension, Fairy Earrings, Pearl Rouge, and Rosetta Ring.

Final Fantasy X-2
Double EXP is an ability found on the Lady Luck Dressphere, and Covenant of Growth Garment Grid by passing through the yellow gate.