Cobweb

The cobweb is a new item introduced in Beta 1.5. It was previously found in the terrain.png file, but had no block ID assigned to it. It is not generated or crafted in any way. Current thoughts are that spiders will nest in it, shoot the web at you to slow you down, or drop it as loot. Cobwebs are also flammable, but they can't be ignited.

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. Unlike water, however, falling into a cobweb will not prevent fall damage, although it will be delayed until you make contact with the ground.

It currently has no use, placing it in a crafting table has no effect, nor is it craftable from String.

History
Notch originally planned for spiders to produce webs.

F.Y.I's

 * Cobweb blocks can be used as air pockets in the same manner as Sugar Cane.
 * Cobwebs can be placed on cobwebs.
 * 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.
 * Animals that walk into the cobweb will slow down, even spiders.
 * Minecarts treat it as a solid block, like glass mixed with wool.
 * 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.
 * Signs and paintings can be placed on cobwebs.
 * Ghasts will not see players in or behind cobwebs.
 * Cobwebs do not stop chests from being opened.
 * Jumping from as little as one block above a cobweb, onto it, will induce half of a heart in fall damage.
 * A shot Arrow cannot pass through cobwebs and will not slow down.
 * Any kind of mechanism (excluding Dispensers and Note Blocks) can not be placed on top of cobwebs.
 * Players are unable to jump while in a cobweb.
 * Cobwebs can be placed on Sugar Cane and in Water.
 * When falling into a cobweb, if a player gets into a minecart or boat before hitting the ground, they will not take the fall damage until they exit the cart/boat.
 * Cobwebs have proven to be useful particularly in water-filled moats 3 blocks in depth. Adding a layer of cobweb to the bottom of the moat will render any mob (or players) stuck. It is possible to get out (granted you will lose health from a near drowning experience) but most mobs will drown. This is a particularly efficient method of repelling creepers from hiding around the corner of your house entrance.
 * Cobwebs have been observed to cause snow in place of rain in biomes where it is inappropriate on SMP servers. This is most likely a bug.
 * 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.
 * Placing cobwebs on ice will increase the slowing effect of the cobweb.
 * As of 1.6.4, walking into a cobweb at head height will now suffocate the player; however, prior to 1.6.4 cobwebs did not suffocate the player.
 * Cobwebs will slow the time of a player breaking blocks like when in water.
 * Currently cobwebs can be destroyed in one hit and has no texture on most texturepacks
 * As of 1.6.5, cobweb pushes you to the outside of the block when landing on top. And because of that, you can not move through cobweb.
 * Cobwebs can be used as traps as it can suffocate mobs and players as water can. However this may be removed as suffocation from cobwebs are a bug that Notch will fix in a future update.
 * Cobwebs, if placed at the top of a 1x2 hole in a wall, could damage the player.