I'm not a fan of protective case for gadgets (which, as long as they function, are fine the way they are, even with a few scratches). For this reason, the only things I've put on my iPhone 3GS so far, and only when I travel, are the Mophie Juice packs, which add more battery life to the phone.
I just, however, found a new protective case that I actually am going to use more regularly, for the sake of convenience. It's the Case-Mate I.D. case.
Unlike other protective cases that try to be as slim as possible, the I.D. case deliberately bulks out at the phone's back to create a small pouch that's just large enough to fit two credit cards or, in my case, my driver's license and a library card (yes, I read).
Of course, you can swap these out for any two cards you want. The point is this is very convenient, especially when the iPhone is something a lot of us don't want to be separated from. And I would like to stress the inseparable notion, as you definitely don't want to lose the phone and the two important cards attached to it.
Despite the added functionality, the case actually only adds another 2.5 mm (about one-tenth of an inch) of thickness to the phone, compared with other purely protective cases.
Other than that, the Case-Mate I.D. is very much like most protective cases. It comes with an easily attached protective film for the phone's screen and another protective layer for the back of the phone (so the card won't mess up all the fingerprints you've gathered).
The Case-Mate I.D. works with both the iPhone 3G and 3GS. It comes in eight colors and costs $30, which is a little more expensive than other cases that add no extra usability to the phone.