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 cannot currently be crafted.

They were added in Beta 1.8 and are generated in Abandoned Mine Shafts. Near the blue spider spawners, they're created in large amounts. They also spawn 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. Unlike water, however, falling into a cobweb will not prevent fall damage, although it will be delayed until you make contact with the ground.

When cobweb is destroyed by shears, pistons, running water, or a sword, it drops 1 string. Cobwebs are also flammable, but they can't be ignited.

History
Notch originally planned for spiders to produce webs.