Diptic, from Peak Systems, is an iPhone and iPad app that creates interesting diptych and triptych images by combining two or three of your photos into geometric arrangements. Diptic's well-designed interface (which provides nearly flawless execution of a single task), simplicity, and thoughtful balance of features make it a delightful app to use.
The workflow in Diptic isn't complex; you begin by specifying one of five different layouts for your photos. After selecting the layout, each space can either be filled with a photo from your device library, or iPhone users can use their camera to shoot new ones.
Diptic uses the same touch controls that any iOS user will be familiar with in order to transform the photos within their individual frames; you can pan around the image with one finger or zoom in and out by pinching. Tapping on a photo brings up a menu for mirroring or rotating by 90 degrees. The only small disappointment is that you can not rotate images freely; you'll need to plan on fixing issues, like a slightly tilted horizon, in another app before you start Diptic.