From FiftyThree, Paper is a drawing app for iPad. It’s simple, useful, and their business model makes me happy.
As a designer you’re always making concepts and interfaces more human. What doesn’t get as much love is a business model that’s user centered. This is what separates the team behind Paper. You download the app for free and start with a single tool, the pen. From there you can play with the watercolor brush, outline marker, and sketching pencil, but you can’t use them in your drawings until you in-app purchase each individually or as a set. Personally I was sold on watercolor (my favorite art medium). Paid apps on the Apple store must be really compelling or highly recommended to purchase, but here I have all the benefits of the free trial period with an obvious upgrade path. It’s the ultimate in show, don’t tell.
Learning makes selling obsolete, but you probably already knew that.