Cobweb

Cobwebs are blocks which cause slow movement. They are found in Abandoned Mine Shafts, especially dense near their cave spider spawners, and in Strongholds, especially in the libraries.

Moving into a cobweb limits your movement abilities. When you are "inside" the cobweb, you will move at a speed equal to about 15% of the normal walking speed. Additionally, jumping will vibrate the player, akin to jumping in a 2 block high space. Dropping an item into it will make the item stick and gradually sink through it. Like water, players falling into a cobweb will prevent fall damage.

When cobweb is destroyed by shears, pistons, running water, or a sword, it drops 1 string. Cobwebs cannot be obtained, even using Silk Touch, but they are available in creative mode.

Cobwebs are flammable, but they can't be ignited directly.

Slowing effect

 * Items thrown on the cobweb are also slowed and will fall down after about 20 seconds.
 * A block of Sand or Gravel which falls or is shot into a cobweb, will slow down, then be destroyed and become a resource after about 4 seconds. Primed TNT seems to be not affected by this phenomenon, as it doesn't disappear and will eventually explode, destroying the cobweb.
 * Mobs that walk into a cobweb will slow down, excluding spiders and cave spiders.
 * Players are unaffected by cobwebs when in a minecart, except when they are in the way of the cart. This allows creation of a mob-slowing tunnel that can be ridden through with ease, making safe rooms even safer.
 * A shot Arrow will pass through cobwebs without being slowed down.
 * Placing cobwebs on Ice will increase the slowing effect of the cobweb.
 * Cobwebs will slow the time of a player breaking blocks like when in water.
 * It takes exactly 8 seconds to get across a cobweb on top of Soul Sand with ice under, given that you had a running start and did not jump.
 * The slowest non-zero speed in Minecraft is achieved by placing water and a cobweb on soul sand, then placing ice under the soul sand, and finally, walking through it with Sneak Mode (left shift by default) activated, and blocking with a sword after using a potion of slowness.

History
Notch originally planned for spiders to produce webs.

Cobwebs were added to the terrain.png file some time before Beta 1.5, but had no block ID assigned.

Cobweb blocks were introduced in Beta 1.5.

In Beta 1.8, new structures were introduced which included cobwebs: Abandoned Mine Shafts, especially dense near their cave spider spawners, and in Strongholds, especially in the libraries.

Before 12w06a, cobwebs did not completely absorb all the fall damage.

Trivia

 * Cobweb blocks can be used as air pockets in the same manner as Sugar Cane.
 * Cobwebs can be placed on any block, including transparent blocks or even other cobwebs.
 * Signs and Paintings can be placed on cobwebs.
 * Ghasts will not see players in or behind cobwebs.
 * No kind of mechanism (excluding Dispensers and Note Blocks) can be placed on top of cobwebs.
 * In SMP, the particles given off when breaking a cobweb may slow down whilst falling, as they appear before the cobweb block disappears. This also seems to cause them not to fly outwards, but just drop directly downwards after the cobweb disappears. This phenomenon only seems to happen if there is a considerable amount of lag on the server.
 * Destroying a cobweb makes "Stone" sounds.
 * While traveling down through a cobweb and attacking, it counts towards critical hits the same as jumping while attacking would - even if you did not jump into the cobweb.
 * As Mineshafts have cobwebs inside them in sparse amounts, finding a room inside the Mineshaft almost completely full of cobwebs indicates that the room contains a Cave Spider spawner.
 * When moved by a piston, or sticky piston, the cobweb will drop as a string.
 * When a cobweb is placed in water, it is destroyed and drops a single piece of string.