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.
Level Lust is an ability gained from an augment found in the Hummingway Home in the DS version of the game. It is a support ability that increases the amount of EXP gained from battles. It can be obtained multiple times via the New Game Plus feature.
EXP Up is a Level 2 ability found only in the GBA version, and is part of the Cannoneer job class. The ability allows characters to gain 1.5x more EXP after battles, and requires 150 ABP to master.
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. It doubles the amount of experience you gain from battle.
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 Meteor is summoned at the Gold Saucer, in the Wonder Square, for 2000 GP.
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. It increases the experience earned by 1.5.
Knowledge to Power is an ability learned by Ashe that increases the amount of EXP gained, and is learned from the Embroidered Tippet accessory.
Accrue EXP is an ability that allows a character to gain EXP as they move. It is learned from the Arithmetician class for 400 JP. EXP Boost is a support ability that increases the amount of EXP earned during battle.