iPhone: There are scads of photography resources on bookstore shelves and on the web, but they're not always available when you're about to shoot a birthday cake scene in pitch black. Nikon's got an app stuffed with portable photography advice.
Of course, if you're somewhere in mobile range, the web's photography resources are, in fact, available on your iPhone. Chances are, though, they're not as organised and immediately on-topic as the information held inside Learn and Explore. Read up on adjusting depth of field, shutter speed, ISO levels, and other fancy-schmancy settings for your digicam or DSLR, and check out examples of what can be done in the situation you're shooting. If you've got a bit of idle time, you can also read the last four issues of Nikon World magazine, and check out the slideshows normally linked to the mag's features on the web.
Neat, free stuff, and it obviously helps any brand of camera.
Learn and Explore for the iPhone [Nikon USA]