iOS: Fans of their iPhone's camera will love social networking app Instagram, a simple "life-sharing" app based on photo feeds. Take pictures with your phone, share them with Instagram and other social networks, and follow a feed of other people's photo updates.
The idea behind Instagram seems to be less archive-oriented and more feed-oriented. That is, instead of something like Flickr, which acts as a repository for your photo collection, Instagram is an app from which you share the random photos you snap on your phone from day to day. You can add effects, share your photos with your Instagram followers, and even upload them to other networks like Twitter, Tumblr, Flickr and Facebook. You can even have it check you into Foursquare when you upload a location-enabled photo. It's actually a pretty neat app, even if you don't have any friends on it—browsing through the "Popular" feed is entertaining in its own right. Hit the link to check it out.
Instagram is a free download for iOS, and supports cameras on the 3G, 3GS and iPhone 4.