Picnic is an app that can be described in one sentence:

Every day, you see all of the photos you took on that day in previous years, and you decide which ones you want to keep and which ones you want to permanently delete in order to save up space.

The interface is nice, though nothing brilliant — it is simply a good idea executed well. They monetize (cleverly, but not too cleverly) by only letting you sift through 100 photos per day, and if you want to do any more than that, you need to pay something like $5 a month or a heavily discounted annual charge. I can't imagine the kind of person who intentionally uses Picnic for more than a year, but who cares? It has provided me value, both at the practical and sentimental level. It feels very much like a 2014 iOS app in the best possible way: humble intentions, obvious value, and a tiny little bit of whimsy.