There are so many filter apps out there but this one hits me right in the sweet spot. Highly customizable, save-able, share-able filters. This is especially good if you want to dial in a signature "look" to your photos. The creator (Drew Wilson) has made a ton of really high quality products, and I've loved every one of them.
His blog post on his 2 year journey to build this: http://blog.drewwilson.com/build...
Koan