We sell two styles of oyster knife in the store: a Dexter Russel with a long blade and a generic short handled knife with a short blade. The size of the blade you need depends on the size of the oyster you're attacking: bigger oysters need a bigger blade. Our shucking crew uses Dexter Russel knives sharpened to a dagger's point. At oyster tasting events we stick with the short-handled blades... they're all the horsepower you need for an extra small oyster. The collection pictured below was assembled by the folks at Oyster Aficionado. We found it on Rowan Jacobsen's blog.
Over at chowhound folks have been discussing oyster knives and their relative merits. Curved blade or flat? Short handle or long? Wide or narrow? Here's our advice on oyster-opening tools: stick to what you're used to, don't buy an expensive knife if you're not going to use it very often, and whatever you do, never use a folding pocket knife.