Cobweb

Cobwebs are blocks which cause slow movement.

Obtaining
Cobwebs can only be obtained by mining them using a tool with the silk touch enchantment. Otherwise it will drop 1 piece of String.

Natural generation
Cobwebs generate naturally in strongholds and in abandoned mine shafts.

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.

Effects
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:
 * Mobs can spawn inside of cobwebs as if they were air.
 * The slowing effect of the cobweb will increase if it is placed on ice.
 * Cobwebs will slow the time of a player breaking blocks, similar to when in water.
 * Players are unaffected by cobwebs when in a minecart except when the cobwebs are in the way of the cart.
 * 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.
 * Items thrown on a cobweb are slowed and will fall through it after about 24 seconds.
 * Items that fall through a cobweb will not merge with other items of the same type thrown on the ground.
 * A shot arrow will pass through cobwebs without being slowed down.
 * A falling block 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.
 * Like leaves, cobwebs do not have any extra effect on block light, but they do diffuse sky light.
 * Primed TNT can pass through Cobwebs without slowing down if they are shot through fast enough.

Trivia

 * Ender pearls are "caught" by cobwebs, which makes them ideal for stopping players from teleporting over walls.
 * 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.
 * 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.
 * 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. This also happens if water touches them.
 * 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.
 * 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.