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. Up until 12w49a, it was impossible to obtain them on Survival. Now it is with Silk Touch.

Effect
All mobs that move through cobweb will slow down with the exception of spiders and cave spiders. 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. Even falling blocks can be slowed down by cobweb. Like water, players falling into a cobweb will prevent fall damage.

Along with affecting the mobs mobility, cobweb also provides a multitude of other effects:
 * Items thrown on the cobweb are also slowed and will fall down after about 24 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.
 * 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.

Use in Redstone
Cobwebs can be used in redstone as a delayer, this works by using a dispenser into a tower of cobwebs above a pressure plate, this is very space efficient compared to other delay methods and can be easily adjusted for the amount of delay needed

Trivia

 * 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 lava(not water) flowing in your direction 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, walking backwards and blocking with a sword after using a potion of slowness.
 * 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 and attacking as the player falls.
 * 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.
 * You may also use a Sword to destroy a cobweb and get string, but it won't happen instantly.
 * Cobwebs can be used in the Xbox 360 version through the use of Creative Mode. But it is just simply called "web"
 * Cobweb will partially stop blast damage if you and the TNT/Creeper/etc are in the cobweb
 * Cobwebs can slow the movement of particle effects that come off players and mobs.
 * It takes roughly 24 seconds for an item to fall through a cobweb.
 * A player travelling in a minecart (that is on rails) will not be slowed down by the cobwebs