HP iPrint Photo [App Store] supports printing of 4x6-inch photos directly to supported HP inkjet printers. The free application utilizes Bonjour to automatically locate compatible printers on your network.
Print [App Store] requires a WePrint helper application installed on a laptop or desktop computer configured to print to the desired printer. Print ($2.99) supports manual configuration to allow the iPhone to print over cellular networks or non-local Wi-Fi networks, and allows printing of photos, web pages and contacts. An updated version, Print & Share, is on the way and claims to support printing of e-mails and files, although due to Apple's restrictions on interfacing with the built-in Mail application, e-mails must be accessed directly through Print & Share.
Air Photo Wifi Printing [App Store] is a new entry into the field that also utilizes a helper application installed on a host computer to permit printing of photos from the iPhone. Air Photo ($1.99) supports auto-rotation of images, as well as cropping and scaling options.