Bugshot is an iPhone app by Marco Arment that is meant for quickly annotating over iPhone screen shots (done by holding the sleep/wake and home button at the same time). It's not an app that you'll use daily or even weekly, but it's perfect when you have a need for it. You can add boxes, arrows, and blur out text (handy for hiding personal information). Once you've finished with the image, you can send it to pretty much any other app (including email and iMessage).
I am a big fan of Skitch for the majority of the things I need to use annotations for (especially actual pictures). There are time when all you need to do is fire off a quick bug report on an App and you can't beat Bugshot's simplicity and speed.