If you're new to DSLR photography or just looking to brush up on your skills then you should take a look at CameraSim for the iPad. The app is a camera simulation that lets you focus on a subject and change common camera settings like shutter speed and aperture to see how each one influences the final picture.
CameraSim launches with a clean UI that has sliders for eight camera settings. In the center of the screen is a sample subject of a girl with a pinwheel at a playground. You can tweak the settings like the lighting, ISO, focal length and more. When you are done, you just press the "snap photo" button to take the picture. The iPad app will then show your resulting shot and offer encouragement if the photo was good or provide suggestions if the photo needs improvement. The app has a decent database of information that'll help you learn about each of these settings, not just change them wildly.
CameraSim is an easy way for shutterbugs to practice their photo techniques without picking up their camera. It's also an excellent tool for future DSLR owners who want pick up some basic photography skills before they make that big purchase. The only drawback I found is the sample photo. There is only one sample and it gets boring fast. I would love to see more sample photos that present a variety of photo-taking situations like a portrait, a landscape shot and so on. It would also be great if you could use the camera to take your own snapshot and use that for practice.
CameraSim for the iPad is available for US$1.99.