A large hammer with two flat faces. Identical to a small sledge hammer with a short handle. They typically weigh around three pounds and have a one to two foot handle.
I have absolutely no idea what they are supposed to be used for, but I found one while digging through a pile of equipment in the RF Engineering Lab of a major Midwestern university. Sitting there, surrounded by the miniature screwdrivers, ultra-fine tweezers, X-acto knives, and dental tools used to work on delicate RF electronics, was this giant sledgehammer. Since I didn't see any railroad ties or house frames anywhere, I can only assume that someone got tired of debugging a particularly difficult piece of hardware.
If it first you don't succeed, get a bigger hammer.
Sears sells these things for about $20.