Silverfish

Silverfish are small, bug-like hostile mobs which have silver skin and black eyes that hide in Monster Eggs found in strongholds. There are three types of Monster Eggs: Stone, Cobblestone, and Stone Brick Monster Eggs. Each of them look identical to normal blocks, but take slightly more time to mine than normal with a pickaxe, and will take noticeably less time to mine without one. When the block is mined, the silverfish pops out and attacks. Destroying the blocks via Creepers or TNT simply kills the silverfish. In regular gameplay, Silverfish are only found in strongholds. If left alone, silverfish can re-enter some blocks, namely cobblestone, regular stone, and stone bricks.

As of 1.0.0 (and possibly earlier), Silverfish can cause damage to a player. The coding behind them allows this to occur when a silverfish is touching a player and a change of the Y-coordinate of either the player or silverfish occurs. This can easily kill a player, since - in a confined space - receiving damage causes the player to perform a short "hop," which counts as a Y-coord change, which allows for the silverfish to damage the player again as the player is coming back down from the initial damage hop. Formerly, Silverfish could not damage the player directly, but would scurry towards the player and push them around, with the tendency to be aggravating and potentially dangerous.

When attacked (but not killed), Silverfish may awaken other Silverfish blocks within a 21x11x21 block area around them. This can lead to a terrifying swarm if there are enough in the vicinity, and an uncontrolled swarm can quickly destroy dozens of stone bricks within a Stronghold. They scurry away after the player moves a short distance.

Spawning
Silverfish spawn from Monster Eggs and monster spawners, which are only found in strongholds. In the latter case, unlike with other monster spawners, they can spawn in any light level, but cannot spawn within a 5 block distance of any player.

History
The texture, silverfish.png, was added in the /mobs folder in the Beta 1.7 update, though the mob itself was not yet present. This was due to Beta 1.7 being most of Jeb's new content at that point for the first part of the Adventure Update, and Silverfish had apparently been completed prior to the release of Beta 1.7 by Jeb.

In Beta 1.8, Silverfish were properly implemented. They could only be found by mining certain blocks with the appearance of Stone Bricks in Strongholds.

As of Beta 1.9 Pre-Release 3, a Silverfish Spawner can be found in the End Portal room in Strongholds.

Before Beta 1.9 pre3, Silverfish had hit points.

In Snapshot 12w17a, the Monster Eggs had names in their tooltips, as since the previous snapshot they were now available using cheats.

Trivia

 * In real life, silverfish are small, wingless insects that are often found in dark areas in buildings. They are infamous for damaging paper-based products.
 * There are also small ray-finned fish called Silverfish. These were proven unrelated after a tweet from Jeb.
 * Monster Eggs are mined at the same speed regardless of the tool used.
 * As of 1.8, they spawn additional Silverfish from nearby Monster Eggs and blocks containing escaped Silverfish when attacked. Blocks are destroyed in the process of spawning.
 * TNT can help avoid this problem since Silverfish hiding in blocks appear to only emerge if the block is broken directly by the player.
 * Silverfish made the same noises as Spiders in 1.8. This was a placeholder and new sounds were made, but not yet implemented at the time.
 * Silverfish do drown in water when forced under by a block above or something similar.
 * Silverfish can be killed with fire, lava or explosions without the Silverfish turning hostile, preventing them from calling in more allies, so long as the player does not hit them directly.
 * Silverfish can ride in minecarts like all mobs but only seem to get into them when jumping, they push them around when just crawling into them.
 * Monster Eggs are often used in some servers as a form of griefing, in which the griefer places them in players' houses, and the player gets attacked by silverfish as soon as they break it. It is also possible to create stairs, bridges or flying platforms out of Monster Eggs. When a player breaks a block (to open a chest, for example), a silverfish will spawn. Inexperienced players will attack the silverfish on sight, resulting in more spawning, which will cause the death or stranding of the player by falling or silverfish attacks.
 * Monster Eggs can be found by almost breaking blocks in stronghold (though actually breaking them is not a good idea) with your hands, as a silverfish blocks break like wool with your hand.
 * Monster Eggs can be pushed and pulled with sticky pistons without breaking. This allows for traps with Monster Eggs exactly where you want them completely legitimately. This is very easy but it takes a long time. To do this you simply place a piston facing a silverfish block (Monster Eggs can be found by trying to mine blocks with your hands) and then operating the piston with a power source (for example a redstone torch or a lever), repeating until the block is where you want it.
 * When the difficulty is set at peaceful, breaking a Monster Egg will make a puff of smoke as it would when spawning a silverfish, but the silverfish despawns instantly, similar to how mob spawners act during peaceful difficulty.
 * Silverfish do not stop and look around like most other land mobs; they constantly scurry around even when not in chase of anything (Tested in Minecraft Beta RC2)
 * If a Silverfish is killed in one hit, such as with a Diamond Sword enchanted with Sharpness I, it will not alert other Silverfish.
 * Silverfish are the only mob which have their own sounds for walking. They make a scuttling noise, regardless of what kind of block they are stood upon.
 * The silverfish, cave spider and spider are the only hostile mobs that are also real animals, however their size is greatly exaggerated.
 * If silverfish hides into cracked, mossy, or circle stone brick, then the block's texture will change into regular stone brick, as they use the same data value and there is no separate type of Monster Egg for the variants of Stone Brick.
 * Silverfish are the smallest mob, chickens being second smallest.
 * In 12w21b update, silverfish stone blocks can be found in the Creative inventory.

Bugs

 * Silverfish will suffocate and can die if they crawl through a Portal block.
 * In multiplayer, even when monsters are turned off, Silverfish will still spawn.
 * In multiplayer, when a silverfish breaks out of a block, the dust effect is not visible.
 * If many pistons are pushing Silverfish then some will go through the pistons.