Our favorite arrow drawing, circle creatin' screenshot annotation app for OS X, Skitch, is now on the iPad. It's pretty much like the desktop version of the app with options to easily draw on pictures, webpages, screenshots, maps or a blanks late. But since you're doing everything with your finger and not a mouse, it's even more fun.
Since Skitch is now owned by do-it-all-note app Evernote, everything you draw can be saved to Evernote for future vieiwing. Sketches are easy to make, with finger, pencil, arrows, text and shape options and can be shared to all the usual social networks and even through AirPlay. The iPad app is free with an iPhone version on the way [iTunes via Evernote]