Skitch allows you to annotate pictures, maps, and web browsers. It is part of the Evernote family of apps.
You can choose a photo from your camera roll, take a snapshot from maps, search using a browser then take a snap and start annotating.
As well as being able to freehand draw, type text you can also pixelate faces or sensitive information.
It's a very useful tool whether you want to use it for brainstorming, pointing out something on a photo or correcting work!