If David Sparks were an app, he would probably be called Workflow. But I know for a fact he isn't an app. I co-wrote a book with him and even had dinner with him in person once. He's a real person. A real, nice person, that is.
At this point, it would be impossible to quantify all of the positive effects David's had on the life of anyone who's touched a piece of software he advocates—even people who have never heard of him.
So fortunately for the creators of the iOS app Workflow, David's not an app, but rather an app-loving human who made an amazing video Field Guide about all the amazing things Workflow can do.
Buy it to learn how to use Workflow. Buy it to enjoy an exceptionally well-produced video. Buy it to support a guy who's probably already given you more than you can pay back.