I’ve had a long love affair with Camtasia (screencasting) and SnagIT (screen capture), but Skitch is a still-image annotation tool that has become one of the more useful tools I’ve ever found. Simply put, it allows you to take an image (camera, screenshot, etc.) and add note balloons, text, ink, and so on. So I can take a picture of the inside of a server and put arrows to the components I’ve just upgraded. I can do an ink line on the route to a building. The sky is the limit. Best of all, it’s still free.