In January 2009, I posted my favorite iTouch/iPhone apps for use in the art classroom. Since then, I've found several new apps to be incredibly useful and engaging for the students in a number of different ways. Note: I have deliberately not included any iPhone apps that require use of the camera. Although some of these products look fantastic, I don't have an iPhone, and thus can't rate them. The apps listed below are free, or a few dollars at most.
Also for photographers, DOFMaster allows you to quickly check your depth of field range for an SLR or DSLR camera.
Finally, I feel compelled to not recommend Photoshop Mobile, because of my extreme disappointment. This application bears nothing that resembles Photoshop except in name.