This free app lets you easily annotate and share images, maps, and ideas. It is part of Evernote, so it is designed to easily help you remember and share your information.
This note taking app includes multiple pen and highlighter tools and functions much like a paper notebook. For $5.99, it offers a lot of organizational and editing tools. Additionally, Noteshelf exports notes as PDF or image files to either Evernote or Dropbox.