Most ultra-durable iPhone cases, like Otterbox's Defender or Case Mate's TANK, are merely water-resistant -- not actually water proof. They might protect an iPhone during a rainstorm, but they can't handle a dunk in the pool or -- perhaps more likely -- a drop in the toilet.
LifeProof's iPhone case, however, promises otherwise. The manufacturer claims the case is Waterproof, Dirtproof, Snowproof, and Shockproof -- the video below demonstrates the waterproof claim. The case completely covers the iPhone, using rubber gaskets around each seal to ensure a tight fit. There is an removable screw that plugs the headphone jack, allowing easy access to that port when necessary. LifeProof also sells an adapter that allows waterproof headphones to be hooked up for use while swimming.
It's not for those who like to switch cases frequently, however. The installation manual warns that the gaskets have a limited lifespan and the case shouldn't be removed and reinstalled more than a few dozen times. Further, the installation process is a bit of a pain -- before it is installed, LifeProof recommends that the case be put underwater for an hour, to ensure that there are no leaks in the gaskets.
The $79 LifeProof case comes in black, white, pink and purple. It is available from LifeProof's online store.