If you've used a Canon printer, you're probably familiar with the bundled Easy-PhotoPrint software. They've now streamlined it for the iPhone interface, and the free app works with Canon's newest wireless multifunction PIXMA printers.
HP has had its own simple but effective iPrint app (iTunes Link) for a while now, and from first glance, Canon's app looks to at least be as good, if not a touch more mature.
The polished interface lets you select paper sizes, find wireless printers, print borderless photos, and select photos from multiple albums stored on your iPhone or iPod touch.
The main drawback: support is currently limited to Canon's MP990, MP640, and MP560 MFPs. It would be nice if any Canon printer hooked up to a Wi-Fi network also worked, but we'll have to wait and see. [iTunes Store | Canon]