A major user complaint about the Canon PowerShot S90 and its followup, the S95, wasn't about image quality, but about how slippery they are. There's at least one custom grip out there that I know about, but if you don't need something quite so special, check out Flipbac's little stick-on grips.
Made from silicone, the inexpensive grips can be attached to just about any point-and-shoot. And because they are silicone, they bend easily allowing you to wrap them around just about any camera body.
Slapping one on your camera is a peel-and-stick affair, so within seconds you can have a better grip and handling. Despite the use of a no-mess 3M adhesive, these things are very secure once applied and do not come off without a fight. You can get them off without damaging your camera, however, so if you want to remove and reuse, you can.
I tested out all three and my preference is for the G2 because of the nice curve for wrapping my middle finger around. Also, though these are primarily made for cameras, I stuck one on an old case for my Droid Incredible giving me a better hold for shooting photos and videos as well as for gaming. The grip is only about a quarter of an inch high, so you can still slip it in a pocket without an awkward bulge.
Flipbac has another cool accessory worth checking out, too, with its angle viewfinder. It's basically a hinged mirror that can be opened 180 degrees horizontally or vertically. For those of you who wish you had an articulating LCD on your camera, this is an easy way to get it. Plus, when it's closed it protects your screen.
Available in 2.5- and 3-inch sizes, and it attaches to your camera with the same no-mess 3M adhesive as the grips. The biggest issue with it is that it's a mirror image, so you'll have to take that into consideration when framing. Otherwise it's an inexpensive way to get ground level, waist level, or overhead shots.
The grips and angle viewfinder are available directly from Flipbac. The grips are $9.99 each or all three for $19.99 and the angle viewfinder is $19.99.