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. Prior to 12w49a, it was impossible to obtain them legitimately in Survival. Now it is possible with shears enchanted with Silk Touch.

Effect
All mobs that move through cobwebs will slow down with the exceptions of spiders and cave spiders. Mobs falling through cobwebs will take 13 seconds to move down by 1 block.

Moving into a cobweb limits the Player's movement abilities as well. When you are "inside" the cobweb, you will move at a speed of about 15% of the normal walking speed. Additionally, jumping will vibrate the player akin to jumping in a 2-block-high space. Like water, cobwebs will prevent players falling into it from taking fall damage, meaning a fatal fall can be mitigated with cobwebs.

Along with affecting the mobs' mobility, cobwebs also provide a multitude of other effects:
 * Items thrown on the cobweb are slowed and will fall after about 24 seconds.
 * A falling block such as sand or gravel which falls or is shot into a cobweb will slow down and 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 the cobwebs 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 minecart with no players in it takes about 9 seconds to fall through a cobweb if a rail is placed directly under the cobweb, but if there is air between the cobweb and the rail, the minecart takes about 34 seconds to fall. Every extra cobweb added will give an additional 25 seconds of falling time.
 * 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, similar to when in water.
 * Items that fall through a cobweb will not merge with other items thrown on the ground of the same type.
 * Mobs can spawn inside of cobwebs as if they were air.

Use in Redstone


Cobwebs can be used in redstone as a delay by using a dispenser or dropper to discharge an item into a tower of cobwebs above a pressure plate or hopper. This is very space-efficient compared to other delay methods and can be easily adjusted for the amount of delay needed. It is also less demanding on the server, reducing lag for that region. As hoppers can take items from a little under a block above them, a two-cobweb tower takes about 5 seconds. Larger towers can use the formula  for the seconds delay. Note that if you have limited supplies of cobweb, the last cobweb, immediately above the hopper, is almost irrelevant, and the first cobweb (in front of the dispenser/dropper) is only worth 5 seconds. Using a pressure plate instead of a hopper adds an extra 10 seconds delay. It is also effectively irrelevant whether the delivery mechanism is discharging horizontally or vertically; a vertical discharge adds a fraction of a second extra delay.

They can also be used to reduce the amount of daylight that reaches a daylight sensor. For example, make two chimneys with a daylight sensor at the bottom; one with a cobweb inside it and one with a cobweb above it. The one with the cobweb above it will be triggered first at dark and last at dawn. This effect is caused by the light mechanics. When it is placed inside, it only reduces the light in one direction (from above) when it is placed above it reduces the light in five directions (from the sides and above).

Trivia

 * Ender pearls are "caught" by cobwebs, which makes them ideal for stopping players from teleporting over walls.
 * 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 and a cobweb on soul sand, then placing ice under the soul sand, and finally, walking through it with Sneak Mode activated, walking backward and blocking with a sword in a slowness potion effect amplified by 5 using the command.
 * Cobweb blocks can be used as air pockets in the same manner as sugar cane.
 * Signs and paintings can be placed on cobwebs.
 * Ghasts will not see players in or behind cobwebs.
 * No kind of mechanism (excluding dispensers, Note Blocks, and droppers) 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 and counts as 2 uses.
 * 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.
 * A player traveling in a minecart (that is on rails) will not be slowed down by the cobwebs
 * Cobwebs can be used for decorative effects, including smoke rising from chimneys and nets for tennis courts.
 * Primed TNT can pass through Cobwebs without slowing down if they are shot through fast enough.
 * Zombies will not catch fire (even during the day) if they are caught in a Cobweb. This occurs regardless of which part of the Zombie caught inside of it.