I can't comment on the gripper. I've got a few Captains of Crush grippers and a few similar ones, but I haven't been using them lately.
Towels are good. I've done towel pullups and I've seen someone doing towel barbell shrugs. Ropes work the same way - pullups from two thick ropes, for example, really challenges the grip.
So are fat bars - I've done lots of thick-handled and fat bar pressing and pulling lately. Even barbell pushups off the fat bar instead of a normal bar. I actually feel like the thick bar makes some of these easier - I really grip the bar tightly when it's 2" or so in diameter.
My gym has a pair of Tyler Grips (http://www.tylergrip.com/
) (W/FS) - I've used them for dumbbell exercises and for pullups on the rack's pullup bar. The softness makes them a little easier on the hands, but the thickness is enough to make it really challenging to keep your grip on the bar. They aren't cheap, since you can use them on any 1" thick handle they're really versatile compared to buying a dedicated thick barbell or DB.
Adding a thicker handle makes it harder to do most of the pulling exercises, and the forearms and grip really feel it.
And oh yeah, rice digs are really good for your hands if they're getting beaten up from training. They are harder than they look, but also more fun than they look. Just imagine you're in that panoramic shot from "Enter the Dragon."
Hope that helps,