I was sent an invite to try Skitch. Skitch is a downloadable Mac app for taking pictures (via screen grab or from web cam) or adding pictures to via drag and drop; then annotating or doodling on the pictures, before sharing the resultant image via web service or simply dragging to email or desktop.
The application’s interface is incredibly slick and it is easily one of the most straightforward UI’s I have ever come across.
There is a video demo of it here
Skitch is in private beta at the moment but you can register your interest here.