Skitch is an image mark up tool that has obvious fun features. However, the ability to add information to pictures and images and share them through Evernote for example, makes it particularly useful for mobile staff and people working in groups on projects.
The desktop version already has fans and the mobile alternative is a good representation of the main release. As well as working well with Evernote, actually its parent firm now, Skitch is also good for sharing marked up screenshots or images via other social networking services like Twitter.
The app is free, easy to use, fun and useful.