You can have spikes added to your armor, which allow you to deal extra piercing damage on a successful grapple attack. If you are not proficient with martial weapons, you take a –4 penalty on grapple checks when you try to use armor spikes.
You can also make a regular melee attack (or off-hand attack) with the spikes, and they count as a light weapon in this case. You can’t also make an attack with armor spikes if you have already made an attack with another off-hand weapon, and vice versa.
An enhancement bonus to a suit of armor does not improve the spikes’ effectiveness, but the spikes can be made into magic weapons in their own right.