Seeing as I take 90% of my photos on the iPhone, I really like the prospect of printing shots right from the device. And coincidentally, that's just what Air Photo can make happen.
You load the Air Photo Server app onto your Mac or PC and the Air Photo app ($2) on to your iPhone. You can then browse your shots through the program and print anything you'd like with one click. I don't print a lot of shots, but Air Photo is such a good, simple idea that I wish* the iPhone could already do this natively.