Rock-climbing requires insane grip strength.
I used to use these a lot when I climbed more often. They work pretty dandy.
Other than getting a pair of those the two best excercise for grip strength are straight-bar hangs. When you finish your pull-ups (palms facing out) then just hang on to the bar as long as you can without falling off. This will build strength and endurance.
For pure strength, my climber buddy used to to do "finger curls". No shit. He would just hold a dumbell (usually 50-60 pounds) and roll the dumbell down to the tips of his fingers without dropping it, then roll it back up.