Piston

Pistons are blocks capable of pushing adjacent blocks, depending on the direction they face. The original version of this feature was a mod posted on the Minecraft Forums by Hippoplatimus. Pistons may be added in Beta 1.5, but Notch said it might get postponed until a later update.

Usage
The original piston mod was capable of moving adjacent blocks, depending on the direction the piston is oriented. To change the orientation, a user would only have to right click on it, and the piston would face them.

Gravel and sand react differently than other blocks, when placed on an upwards-facing piston. Instead of simply riding the piston up, they will launch into the air, before falling back down into place. Other things that can be launched are players and items (items may also be launched sideways).

Pistons are controlled using redstone currents.