Chris D. over at Strength News has just received his Captains of Crush Trainer Gripper by IronMind Enterprises and seems well pleased with them. They are basically the same as grippers you would buy at a sporting goods store, except that they require a lot more effort to close. IronMind has them available in 60 lb up to 485 lb strength (apparently, only 5 people have been able to close the 485 lb grippers under certified conditions). The basic ones are 100 lbs.

Pop over to Strength News if you have any advice on how he should work them into his workout routines. I have normal ones that I just use as a stress relief or to occupy my hands while watching the babble box.