Monster Spawner

Monster Spawners (AKA Mob Spawners) are cage-like blocks that were added in Update 0.9.0.

Use
Monster Spawners spawn Mobs around them in a certain radius (3 blocks) depending on the mob they are programmed to spawn. A small image of the mob on fire will rotate inside the cage, and will gradually get bigger, until the mob spawns.

Locating
Mob Spawners are mostly found in Dungeons, Strongholds, Nether Fortresses, and Abandoned Mineshafts, accompanied by Chests containing various items and are presumed to challenge the players that try to raid the chests.

Disabling
Monster Spawners can be disabled by placing 1 Torch on top of it, or placing 4 Torches on its sides. Monster Spawners can also be disabled by breaking them.

Trivia

 * As of Update 0.9.0, it will spawn Hostile Mobs in Survival Mode, in Dungeons, Strongholds, and Mineshafts. Dungeons will have Zombie, Skeleton, or Spider spawners. Strongholds will have Silverfish Spawners. Mineshafts and Mesa Mineshafts will contain Cave Spider Spawners.
 * In Creative Mode, it is possible to change the mob spawned by a monster spawner using Spawn Eggs.
 * Dungeon spawners have a higher chance of spawning Zombies  (50%) compared to Skeletons and Spiders (25% each).
 * Torches will not affect Spawners present in the Nether.
 * Mobs spinning inside the spawner can wear Armor, but it will not affect the chances of a mob spawning wearing Armor.
 * Monster Spawners drop Experience Orbs upon being destroyed.