Spike Block

Spike Blocks are blocks originally planned to be introduced instead of Sticky Pistons. They would work like pistons but push out Spikes. It was later suggested to Jeb that he should make Sticky Pistons instead and also make a Spike Block that could be pushed by them.

Uses
Presumably, the Spike Block/Piston would have been used in traps, therefore they would probably have caused damage to mobs and the Player.

History
On May 28, 2012, Dinnerbone stated he wishes to add Spikes to Minecraft, but is unsure how to implement them.