Cobwebs

The Cobweb is a block added in Beta 1.5. When first added they were found in the .png files but since they had no block ID assigned to it, they weren't actually added until Beta 1.8.

When a player walks into a cobweb it causes slow movement. Cobwebs will naturally spawn in underground abandoned mine shafts and strongholds.

Every single mob and dropped items in the game are affected by the cobwebs slow movement effect, exept Spiders and Cave Spiders and when movement vertically down Cobwebs it takes about 12 seconds to move through each block. When moving through a Cobweb your walking Speed is reduced to 15%, speed potion will effect this. In Creative Mode while flying, the player doesn't get affected by the cobwebs stickiness.

Cobwebs can also be used in Redstone as a delay by using a Dispencer or Dropper to discharge any item into a cobweb stack. Underneath the cobweb tower could be a either a Hopper or Pressure Plate to catch the item and send a redstone signal of. This use of a delay system is much more compact and arguably much more cheaper.