Traditional martial arts practitioners would perform exercises intended to harden their fists into even more impressive weapons. Although these exercises are less common -- and often wildly inappropriate -- in the modern world, they are worth knowing about if only for historical context. A hard fist is worth having, but not as valuable today as fingers that can hold a pen or work a keyboard. Many of the traditional exercises relied on abuse to the hand that left them inflexible and insensitive.
Hitting a punching bag repeatedly with no gloves or hand wraps can help condition your hand for punching. Although this does physically harden your hand, its chief benefit is in improving your punching technique so the hand hits harder and strikes with the strongest parts of your hand. In terms of actual hardness, bag work will quickly develop callouses on whatever surfaces you use to strike the bag.
A makiwara board is a thin sheet of canvas -- sometimes with a bit of padding beneath -- spread over a wooden board. Martial artists strike the makiwara with various hand forms, conditioning themselves against striking such a hard surface. In addition to developing callouses, repeated makiwara work will cause microscopic tears in your muscles and fractures in your bones. These will heal back thicker and stronger, much like a scar is stronger than the skin around it. However, also like a scar, the healed area will be less flexible and sensitive.
Ash and Stone
Traditional martial artists would inure their hands to pain by plunging them into basins of sharp pebbles or hot ash. This improved their tolerance for pain, covered the entire hand with thick callouses, and damaged nerve endings to reduce sensitivity. Although these practices do a fine job of turning a person's hands into a strong blunt instruments, they do an even better job of rendering them useless for tasks that require fine manipulation.
Iron Palm Oils
You can find iron palm oils or liniments in most martial arts supply catalogs. These liniments use herbs that improve circulation in the hand while simultaneously conditioning the skin outside. Although not technically an exercise, these oils can help harden your hands. They can provide some of the benefits of fist-hardening techniques while avoiding most of the problems.