Recently I've been having a bit of a renaissance of creative activity with my iPhone, thanks to Instagram, an addicting photo sharing app. It seems to hit a design sweet spot between simplicity and functionality. The app comes packaged with a finite set of predefined filters, so when you take a photo, you can quickly decide what transformation is the best fit. For some people this might be an undesirable limitation, but I prefer to have to work within the constraints of a few canonical forms, and develop an intuition about what filter works best for a given shot. The app also provides an in-app-only (i.e. no web version) social networking component which is a lot of fun, providing you a feed showing photos from everyone you are following. It's basically a mechanism for curating interesting photos and provides inspiration and ideas. I'm finding myself being more observant of my surroundings, looking for interesting arrangements of objects, lighting, textures, etc that would make an artful composition to capture. The app also allows you to upload your photos to Facebook, twitter, flickr, tumbler, etc. I highly recommend it!
Gizmodo posted a very favorable review of Instagram a couple days ago: The Revolution Will Be Instagrammed by Matt Buchanan.