I had a chance to do some pullups and holds using a training board today. My forearms were on fire. Not exactly a grip specific tool, but it would definitely increase your grip strength.
On a similar note, one of my favorite grip exercises is I have a set of climbing holds mounted across a rafter. Doing pullups with a pinch grip or even just hanging with a pinch grip really tests your grip. After your first set, the rest are very short.