Finetuna is a dead simple collaborative image annotation tool. Meant mostly for casual designers, it lets you make a few short notes on an image and send it off to someone else. As soon as they get it, they can view your edits, make their own, then send it back. This keeps the paper trail out of your e-mail in-box, and in a single place.
Finetuna's key appeal is that it does several simple things with ease. You can highlight, underline, and cross out text. There are also simple tools to insert text and make comments, which are essentially the same thing; you just get a box to write in suggestions to the sender. There's no free-form doodling tool to make mark-ups, which photographers might yearn for, but the highlighter comes close.
There are a handful of ways to get images into Finetuna, although the easiest is just to upload the shot from your computer. You can also drop in a direct URL from where the image is hosted, or download the Firefox extension which lets you mark up an image from any page you're on.
I'm not sold on professionals using this over tools like ConceptShare and ProofHQ, but if your basic needs are highlighting, making small comments, and you don't want to sign up for anything, it's tremendous.