Skitch, a popular annotation app for Android and Mac has come to the iPad. It is free and one of the most useful apps in my honest opinion.
Ever felt the need for an app that lets you explain your ideas to people by drawing on the screen, typing and creating arrows? Skitch is the perfect app for the job. It can annotate any image and send it off.
You can directly take snippets from a map or webpage from within the app or just import a photo of anything (even screenshots) and start drawing on it. You can create shapes, beautiful arrows and put in text. You can save it or send the image off to someone using Twitter, email and more.
Skitch also integrates with Evernote if you like (it was recently acquired by the company). You are not forced to use the service, though.
Such simple annotation is what the iPad was made for. However, Skitch cannot annotate on anything except for images. Don't expect PDF annotation or anything like that from it. But you can take screenshots of pretty much anything, though.
Get the free (and devoid of in-app purchases) Skitch for iPad here.