I got a good one I used at the end of today’s workout. Get a wash-cloth and with both hands just squeeze as hard as you humanly can for 30 seconds. It sounds so nice and easy but after a full workout when your grip is fatigued it is absolutely nasty.
Barbell holds will give you endurance. Just load up a barbell, pick it up and hold it for as long as you can. Use palms out and in grips.
http://www.sandowplus.co.uk/Competition/Coulter/Stunts/stunts-intro-1.htm
A complete book for tearing a telephone book, a deck of cards in halves, to bend a heavy steel spike with the hands, etc.
It becomes a little bit outdated, but it could be useful.