Experience Booster

In Video Games, there are items, abilities, etc. that help players gain more Experience Points, especially when Level Grinding is a problem. In most games that these appear in, experienced gamers will try to earn and equip these as soon as they can, since they can usually help raise all other stats more quickly.

Compare Leaked Experience, if it happens by default; and Rare Candy, which provides an instant boost in stats.

Tabletop Games

 * Dungeons and Dragons
 * In early editions:
 * Having a high value in the correct ability score increased earned Experience Points by 10%.
 * There were magic items that could increase the number of Experience Points gained by the owner or permanently increase a character's level.

Video Games

 * Super Mario RPG has an item straight-up called the Exp. Booster. Whoever equipped it gained double EXP from each fight.
 * The Disgaea series has the Statistician specialist, which can be placed in a piece of equipment to provide as much as four times the normal experience. Couple this with the fact that you can stack as many of them in your equipment as there are spaces available for them, and it becomes possible to hit the level cap of 9999 in a few battles.
 * The first game also handed out statisticians like candy, so this was pretty easy to exploit. Later games typically make them much harder to find.
 * Some Final Fantasy games have an experience-modifying item.
 * Final Fantasy VI called it the "Exp.Egg", doubling the user's experience gain.
 * Certain weapons in Final Fantasy VII had Double (or even Triple) the rate of AP gain (which is more important than EXP in the endgame). Not to mention the literal "Exp. Plus Materia" that increased the exp. earned by anyone who equipped it (and could itself be placed in AP+ plus weapon to replicate faster and be distributed to all party members).
 * Certain weapons in Final Fantasy X also had double AP gain.
 * In Final Fantasy XII, the Golden Amulet doubles the LP gained, and the Embroidered Tippet doubles EXP.
 * Final Fantasy Tactics has the "JP Up" ability that allows a character to gain more Job Points in battle.
 * Final Fantasy V Advance-exclusive jobs Cannoneer and Oracle have abilities that do this, with the Cannoneer's a more orthodox variant and the Oracle's one more in the vein of JP Up.
 * The Exp. All (from Pokémon Red and Blue/Yellow only) and Exp. Share divides experience gained from battle among your party or a certain Pokémon when held respectively, and Lucky Egg, which doubles the experience gained in battle, and rarely held by Chansey. Traded Pokemon also get an increased experience boost.
 * Pass Powers, introduced in Gen V, include increasing or decreasing experience gained by a little or a lot.
 * The Macho Brace, Power Weight, Power Bracer, Power Belt, Power Lens, Power Band, and Power Anklet affect Effort Value gains after battle, with the first doubling all, and the rest adds 4 to a particular Effort Value.
 * The Pomeg, Kelpsy, Qualot, Hondew, Grepa, and Tamato Berries decrease Effort Values of a particular stat.
 * Golden Sun: Killing a monster with the right type of Djinn gets increased experience, money, and chance of randomly dropping an item.
 * In many Flash games where you can buy upgrades (shooters, Tower Defenses, RPGs, etc.), there is often an item that gets extra experience (or money) per kill, very often the first that should be obtained.
 * By giving them the proper titles, any item from Phantom Brave may become this.
 * In a New Game+ of Mass Effect 2, the entire squad received 25% more experience.
 * While Dungeon Siege has no experience points, items that artificially raised your skills and had you use them (like an axe that raised your melee skill) fulfilled the Experience Modifier role: having higher modified skill meant you landed more hits, and landing more hits raised the base skill value faster, and, of course, the modified skill value rose as well.
 * World of Warcraft has Rested EXP. When you log off in an Inn or major friendly city, you build an overlapping meter in which you get double EXP from kills until you hit that point. You wouldn't get boosted EXP from other sources (such as quests), but you wouldn't 'lose' Rested EXP in those cases either (the "rest" bar would adjust itself upward to compensate).
 * One background in Arcanum ("Nietzsche Poster Child") increases EXP gain by 10% while giving the character a noticeable boost to critical failure rate ("That which does not kill me can only make me stronger."). Unless you are going for a Pacifist Run (as diplomats don't make rolls that can critically fail, but do have limited sources of EXP, even then a permanent boost to your relevant stats, with an increase in cap, in-exchange for blows to combat ones is also a option), it's not worth it,.
 * In the Persona games, there are two types of experience modifiers: Social Links and from after-battle Shuffles. The Shuffles are cards you can earn after a battle that automatically raise how much experience is earned from the battle or randomly raise a persona's stats. Social Links, the higher they are, give bonus experience when creating personae through fusion according to the corresponding Social Link/Arcana.
 * Runescape has all sorts of experience boosters. The Runescape Wiki has a list.
 * Dungeons and Dragons Online has the Voice of the Master or Mantle of the Worldshaper that give a 5% bonus, potions for up to a 30% bonus, Tomes of Learning for up to a 20% bonus, and Experience Shrines for up to 5%. All of these stack.
 * In Too Human, as your combo meter raises you get extra xp (max of 3% at level 3 combo meter) Runes can also be fitted into weapons and armor that also boost the amount of xp gained (stackable up to a certain amount, unsure of the cap)
 * Aion also has rest xp as well as bonus xp that starts to show up if you are leveling too slowly
 * The World Ends With You has several items that do this.
 * The Scripture in La-Mulana. It's arguable whether or not the experience boost justifies its exorbitant price.
 * Fire Emblem has the Paragon/Elite skill, which doubles EXP gained.
 * Sometimes this skill halves EXP gained, but gives more stat increases for level ups. The net effect is that it takes around as long to get their stats up, but they're in less danger of maxing out their level.
 * The Tales (series) has the Dark Seal, which multiplies EXP earned by 1.5x but increases damage taken and inflicts the Curse status ailment on the party member who has it equipped, and the Demon's Seal, which doubles EXP earned but inflicts various status effects. The latter is very useful in combination with the Yasakani Jewel, which blocks all status effects including those caused by the Demon's Seal.
 * The Inazuma Eleven games, starting with 2, have a passive skill named "gakushuu" (study), which multiplies EXP earned by 1.5x for individual characters who have it. A select few characters come with it already learned, but for the vast majority, it'll cost you 30,000 Nekketsu Points to teach it to them. Like other passive skills, it also takes up one of the 6 hissatsu technique slots each player has - you can overwrite it, but if you want it back later, it'll cost you another 30,000 Nekketsu Points to re-teach the skill.
 * Your mentor Kreia has this effect when she's in the party in Knights of the Old Republic 2.
 * In the Castlevania series, there are items (and even soul abilities in the Sorrow games) that have this effect. They are not that useful, however - level grinding isn't as important as having good weapons, abilities, and playing the games better.
 * The Fallout series has the Swift Learner perk, which grants a bonus of 5% (1, 2, Tactics) or 10% (3, New Vegas) to experience gained.
 * Diablo II has experience shrines randomly scattered in some areas, which increase your experience gain from slaying monsters for as long as the shrine's blessing remains active.
 * Diablo III also has experience shrines, as well as experience boosts from equipment.
 * Heroes of Might and Magic has Learning skill, which does exactly this. Its version from V, Enlightenment, also gives the hero extra stat points every several levels (retroactively), becoming a major Game Breaker in long games.
 * One of the possible unit upgrades in Civilization IV increases the experience gained in future combats.
 * The Super Robot Wars games have the "Gain" and "Cheer" Spirit Commands, which double experience gained from one fight. Cheer can be cast on any friendly unit, but Gain only affects the caster. The most obvious use is finishing off a boss.
 * In Dark Souls, the Covetous Silver Serpent Ring and the Symbol of Avarice helm.
 * Zilean in League of Legends acts as one of these for everyone on his team.
 * In Dragon Quest IX, the Luminary's Coup de Grâce gives a random experience boost to all party members.
 * In Odin Sphere, you can buy a Spirit Gem from the very first merchant you encounter, after the first stage of the game. Equipping this item increases the experience that your Psypher Weapon gains when you absorb phozons. You don't need to have it equipped all of the time, so you can just swap it on whenever you're done killing a wave (or entire level) of enemies. It's also a handy part of the Phozon Farming Abuse trick, which involves using the Phozon Release skill to turn some of your "magic points" into free-floating phozons. Equip the Spirit Gem, release all of the phozons you're carrying, and then reabsorb the phozons. This restores your MP gauge (though not to the same level as before), allowing you release and reabsorb all of your phozons again and again. Doing this often will grind up your Psypher Level, especially with the Gem.
 * In Dungeons of Dredmor, Big Game Hunters have a chance to earn a bit more experience when killing enemies that are animals, and Archaeologists can get a lump sum of experience by sending enchanted items to the crate room from Indiana Jones. Yeah, it's that sort of game.
 * In Spiral Knights, Heat Amplifiers temporarily double the amount of heat you earn for your gear. Activating a new one adds its duration to the current amplifier's.
 * The Kingdom of Loathing's various monster aggravation devices and items make monsters harder, and thus worth more XP. Achieving the balance between more XP and not getting killed by +50ML or more a key strategy in speed runs.  There are some items that give stats per adventure directly, but they're not as good.