Some apps apply a vintage filter on your snapshots, and others let you share your snapshots with friends. Instagram does both, taking mobile photography social. Instagram's defining feature is its feed. You can "follow" your friends and various media outlets, and if they follow you back your shared shots appear on the feed for everyone's viewing and commenting pleasure.
Filling up your new iPhone? Just hoping to make the one you already have even better? These are the apps you don't want to be without.