Trapdoor

The trapdoor is a new block type implemented in the beta 1.6 update.

They are intended to be placed over a single-block hole (but can be placed over a block as well) with a solid block directly adjacent. When activated, they will then open by flipping upwards toward the adjacent block. Just like doors, trapdoors can be activated by redstone. If that block is destroyed, the trapdoor is as well. When placed outside, rain and water do not pass through a trapdoor.

Crafting
When crafted with six wooden planks, two trapdoors are created.

Trivia
Much like the piston is based on a mod developed by Hippoplatimus, the hatch is very likely based on a mod from DJoslin. (http://www.djoslin.info/trapdoor.php)