Double-overhand heavy deadlifts are fantastic for developing grip while also getting stronger everywhere else.
I'm a rockclimber, where grip is also an important tool. A lot of people use hangboards and work either pullups or static hangs with their hands in all sorts of positions. It's easy to injure yourself on these, however.
Towel pullups strike me as a good tool, too - those things kill my hands.
On rock? Rock on