Napkin is a brand new take on image annotation crafted for the Mac by Aged & Distilled. Combining intuitive gestures, fluid shape creation, just-type text entry, and a lot of really disciplined, really smart interface choices, it manages to bring the immediacy of iOS to OS X, producing results that are as fast as they are beautiful. We use it for many things at iMore and Mobile Nations, including all of our help and how-to articles, as well as internal design review and feedback. It's just that fast.
Napkin doesn't do everything a full on PDF suite does. It takes a deliberately less-is-more approach. It's reductionist by design. The only possible exception is the menu bar. While Apple has increasingly taken color out of their Mac interfaces, Napkin still uses it, and uses it well. It never competes with the content on the canvas, but is always there to better snap the eye to the controls.
Everything in Napkin is as direct a manipulation as the Mac allows. It's as close to having an iPad app on the Mac as possible, and that also feels very much by design.
Once you get get used to it -- something helped enormously by an introduction video and initial text tips -- you can mark up everything from interface concepts to internet memes with almost no overhead. Napkin just figures out what you're most likely trying to do and just does it for you, letting you fine tune or change the results afterwards, if you need or want to. It stays entirely out of your way unless or until you decide otherwise. That's what makes Napkin so speed-force fast.