This is a product review for one of my favorite gym tools - Tyler Grips. (warning, original homepage is now broken - these are no longer made!)
$39.95 a pair, plus S&H.
Tyler Grips are neoprene cones (with the pointy end chopped off) with a split down the side. They are design to be slid over any 1" barbell, dumbbell, or cable machine handle, instantly turning it into a thick-handled implement.
The grips are comfortable to hold onto. They have some give, enough that you can really give them a good squeeze to hold on. Since they are split, you need to squeeze them to keep them tightly on the bar. It is a killer combination - you can grip them hard and need to, so you always get a good grip workout when you use them.
The additional thickness provided by the Tyler Grips, especially because it's uneven (wider at one end, narrower at the other), makes for good grip work. Because they can be added to anything, you can make grip-intensive variations of your normal exercises. Pullups, dumbbell shugs, snatches, or curls, deadlifts, barbell rows, etc. Just be careful with the weight - what you can thick-handled lift is always lower, often much lower, than you can handle with a normal, 1" thick handle.
They are not a replacement for thick-handled implements, more like a complement to them. A 2" thick Olympic bar, pulldown handle, or pullup bar is still very useful for improving your grip. But the cost of getting a thick version of all of your equipment can be prohibitive. Tyler Grips can make for a good substitute in cases where the "proper" tool isn't available or costs too much. They also make a nice variation from normal thick bar exercises.
Utility: 5 of 5. They're easy to put on, take off, and use, and they substitute for a lot of more expensive equipment.
Quality: 5 of 5. These grips will stand up to a lot of wear-and-tear.
Bottom line: If you want to do a variety of thick-handle work but can't afford a variety of thick handles, go for these.