Trapdoor

Hatches or trapdoors are a new block type not currently in the game. They will function the same as doors, but are oriented such that they will be placed on a flat, horizontal surface. Like doors, they require a solid block next to them to be placed, and also do not allow water to pass through. Unlike doors, hatches only occupy a single block of space, and they open upwards.

Trivia
It probably is, like the piston, based on a mod from DJoslin. (http://www.djoslin.info/trapdoor.php)