Encounter Bait

One type of Standard RPG Items, Encounter Bait is used to attract enemies into Random Encounters, mainly by players interested in Level Grinding.

Magic Is a Monster Magnet is sometimes used for this purpose.

Contrast Encounter Repellant.

Anime and Manga

 * Near the beginning of Bleach Uryu uses a Hollow-summoning bait during a contest with Ichigo to determine who can destroy the most Hollows.

Tabletop Games

 * Dungeons and Dragons: over the years there have been a number of magic items that increase the chance of Random Encounters. However, these are usually considered cursed items because randomly encountered monsters usually have less treasure than normal and the resulting fights either (a) just wear down the PC party for no gain or (b) interfere in the party's current mission.

Video Games

 * In the original The Legend of Zelda, you can purchase monster bait. It's mostly used as a bribe to get past one of the dungeons, but if you use it on any other screen the monsters are drawn to it.
 * There's a variety of ways to increase the rate of wild Pokémon encounters, most of which were added in Pokémon Gold and Silver. The move "Sweet Scent", items like "Sweet Honey" and the "White Flute", or tuning into the Johto Radio station while it's playing the Pokémon March; Pokémon with the "Illuminate" ability also increase the encounter rate, and even using the Bicycle or Running Shoes in games that have them will increase the encounter rate (an early NPC in HeartGold and SoulSilver cautions that "the extra noise will attract them").
 * Traveling in a zig-zag pattern (e.g. up, right, up, left, repeat) is also noted to increase the encounter rate in all generations including the first, because the player character turns to face that direction before taking an actual step. In fact, it is possible to generate Random Encounters on demand by repeatedly tapping different directions on the control pad to turn the player character without taking any steps at all.
 * The Black Map in Skies of Arcadia, which not only increases the Random Encounter rate, but also prevents enemies from running away from battle... very useful for grinding Loopers.
 * Golden Sun has the Lure Cap, which increases the Random Encounter chance. Problem is, it's nowhere near effective as armor at that point in the game.
 * In Red Dead Redemption, you may use bait to attract wild animals.
 * In Nethack you can use the Create Monster spell to create a hostile random monster to kill. However, only wizards who have the wizard quest artifact regenerate Mana quickly enough to make casting this spell over and over and over a viable tactic.
 * The GBA version of Final Fantasy V has the Lure ability for Gladiators.
 * Final Fantasy VII has a monster bait materia.
 * The "Whistle" skill in Dragon Quest games always starts a battle.
 * "Enemy Welcome" and the higher-level "Enemy Challenge" in Shin Megami Tensei Strange Journey. However, they're sub-applications installed on the Demonica, so they remain effective for as long as they're installed.
 * Tradewinds Legends has an item that causes you to encounter fewer storms, but more pirates.
 * Terraria has a few of these items. The most straight example is the water candle that increases spawning rate of monsters when you're currently wiedling it. Then there's the Worm Food which summons the Eater of Worlds boss and the Strange Eye which summons the Eye Of Cthulhu boss.
 * As of the introduction of potions, there is now a potion which greatly increases the spawn rate as well.
 * Final Fantasy IV has Sirens, consumable items which force an encounter right then. Highly valued by completionists, as the encounter forced is fixed and is generally the rarest encounter in the area--which generally have the best treasures available as random drops. They're basically the only way to encounter Flan Princesses/Pink Puffs with any kind of regularity--ridiculously rare critters with ridiculously rare and valuable treasure.
 * Dark Bottles in the Tales (series). Although the encounters are pretty frequent anyhow, they turn the fights from one every five steps to one every two steps in the games that utilize random encounters. In the games with visible monsters, they make defeated monsters respawn much faster.
 * In Zeboyd Games' Breath of Death VII and Cthulhu Saves the World, the "Fight" command in the menu forces your party into a battle if you're outside a town. Since each zone has a finite number of Random Encounters (20 for individual dungeons, and 200 for the World Map), this command can be used even if that cap has been used up, just in case you want to Level Grinding just before a tough boss battle.
 * Space Rangers II has the Transfactor Beacon, an artifact that causes the star system to be attacked by Kelleroid dominators -- useful both for Level Grinding (until a certain patch, at least) and for causing Enemy Civil War.
 * ZanZarah had the Ocean Conch, which you got fairly early in the game but worked only on the water fairies (who were considerably less aggressive than other types), and Fairy Horn, which worked on every wild fairy.