Photographer Anthony Tortoriello chimes in with a few ideas about photo apps. Apps he uses include Camera+, which is great for Macro. It also has the ability to turn on the rule of thirds grid, which is great for keeping horizons level. Photo editing apps include Snapseed by Google. It's great for sharpening and pinpoint adding of contrast and lighting. You can also add captions and watermarks, and touch/retouch. Leo says that Adobe's Lightroom app and Adobe Mix are all great. Tony says that Lightroom on the iPad is fantastic. Instagram is another great photo app.
But the iOS Camera app is really powerful. It has time lapse and slo-mo video as well.