New Skitch PDF Annotation
There are a lot of Evernote users out there. Skitch is an application that works in tandem with Evernote by actually saving all of its data to Evernote. (Evernote owns Skitch)
Skitch has historically been a great screen capture app. It allows you to quickly grab a screenshot, add arrows, circles, or text, and then easily share.
Skitch now has the ability to add "stamps" and annotate PDF's. I could see Skitch being used by teachers who need to provide feedback on a variety of student submitted PDF assignments.
Check it out!