What are your iOS hidden gem apps

Preferably lesser known but game-changing apps that are able to be bought one-time and put all others to shame.

To help clarify your thinking, which apps have produced such an outrageous level of value (regardless of one-time cost) to the extent you believe it should be #1 in its category, not necessarily #1 app ever.

We'll do a seperate thread for Mac, Windows, Linux, Android, etc but let's stick to iOS for this one. Thanks Lemmings!