I have average-sized, adult male hands. Admittedly nit-picking here -- the stock grip will do in a pinch. I've used these:
1. Bushmaster A2. Too skinny, IMO. Gets a little slippery with sweat or inclement weather, though the finger grooves help.
2. Duckbill Tactical Grip. Solves the problem of raw under-the-trigger-finger skin during prolonged shooting, such as in a daylong match or rifle class. But I found the grip got slick fast with sweat and inclement weather, and it was still too slim.
3. Hines Ergo-Grip, "Sure-Grip" model. Rubbery service with a sure purchase, but overall it flexed too much for my taste. Didn't feel "solid" enough to me during gun handling.
4. Hines Ergo-Grip, "Rigid" model. What I'm currently using. Very good ergonomics, grippy surface, solid feeling. You reportedly can sand areas for a custom hand fit. It's still a little on the skinny size, even for my hands, so trigger-finger positioning under minor stress (such as in a match or rifle class, vs. bench shooting) still is not optimum. Drawback: not optimum for ambidextrous use; mine is a right-hand version, though I have shot it support-side OK. I think Hines makes an ambi model.
I've handled these on a few occasions:
1. Sierra Precision grip. Fat one! Hard plastic. Hand-filling, but seems like it would get slippery when wet with sweat or in inclement weather. Probably best for big guys with big hands.
2. Hogue. A handful, but not too much. Same rubbery material as a Hogue revolver grip. Hard, but "gives." Probably going to give this one a try -- I think it might just be the ticket. That or stick with the Hines.
If you're interested in buying any of these, just run a Web search to find online retailers. The Bushmaster or Brownells sites also are good bets.
The Hines "Gapper" is a great, cheap little gizmo that solves the same problem as the Duckbill grip. Goes in, stays in, too. Here's a link, if you're interested:
http://www.gunguide.com/~shines/page17.html