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. Sticky Pistons were introduced in Beta 1.7.

Uses
Presumably, the Spike Block/Piston would have been used in traps, therefore they would probably have caused damage to mobs and the Player. One possible crafting requirement for Spike blocks would be iron bars, since they are already spiky in appearance and would be cheaper to use than iron ingots. If spike blocks were to be used with pistons, the 3 wooden planks could be swapped out for iron bars.

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