I first posted this roundup not long after the iPhone 4 hit stores and it's been through a few updates, and now only includes cases that fit both the iPhone 4 and 4S and carry the "universal fit" tag.
The $79 LifeProof case isn't cheap, but it may be the best all-around tough case (click to enlarge).
Recently added: Case-mate Phantom, Qmadix Xtreme, and Rokform Rokbed Fuzion.
As I've said before, how you choose to protect--or not protect--your iPhone is a matter of personal preference. But if some heavy-duty shielding is what you're looking for, this is the list for you.
While I tried to pick tough, protective cases that look good and don't make your iPhone too bulky, I should point out that a few of the cases have more-extreme designs that aren't necessarily intended for everyday use. I should also note that some of the more-rugged cases can be a bit of a chore to remove and aren't friendly toward iPod docks.