Boots with steel inserts over the toes. They're standard safety clothing in factories and other environments where heavy items may fall on the foot. For added protection, metatarsal protectors may be added to shield the front of the foot.

In addition to normal mayhem as described by Andromache, steel toed boots have a distressing tendancy to be employed in meelee situations against both people and animals. Skinheads are notorious for doing this.

When I took weight training classes, I was instructed not to wear steel toed boots, because dropping weights on the boots could deform the steel into the flesh of the foot. I guess there's just two different schools of thought on the subject.