The "WASP Injector Knife" secrets a CO2 canister in the handle which, when a small button on the hilt is triggered, injects a blast of 850psi gas through a channel in the knife's blade, inflating the target with a painful and debilitating pocket of gaseous pain. It's being marketed towards scuba divers, although I'd be too terrified of accidental discharge to carry it myself, let alone try to get into a cutting match with a shark.
From their product description page:
This weapon injects a frozen ball of compressed gas approximately the size of a basketball at 850psi nearly instantly. The effects of this injection will drop many of the world's largest land predators. The effects of the compressed gas not only cause over-inflation during ascent when used underwater, but also freezes all tissues and organs surrounding the point of injection on land or at sea. When used underwater, the injected gas carries the predator to the surface BEFORE blood is released into the water. Thus giving the diver added protection by diverting other potential predators to the surface.
The knife itself doesn't actually seem to be on sale yet, but prices of around $400 or so were being bandied about before someone claiming to be with the company said they were "in negotiations to sell it strictly on the non-civilian market." So who knows? However you slice it, it's a wicked little weapon.
Product Page [Waspknife.com]
Update: There's a video demonstration now: