I know I'm late to this party since the app has been around for awhile, but Skitch from Plasq just blew me away in about 5 minutes. I wanted to create some quick screenshots for my guide to setting up cross-posts from Vox to LJ but wanted a faster way to do it besides taking screenshots, cropping them in an image editor, drawing on them with a paint program, etc.
I remembered that Plasq had something like that so I went to their site and found Skitch. It's a free download, requiring only that you create an account.
Once it was running and I'd configured my Flickr account in the Webpost settings, I just clicked Snap and dragged a box over the portion of the web page I wanted clipped. It automatically pulled it into the UI, as shown above. I drew my circles and arrows, none of which affects the underlying capture, and when I was done clicked Webpost. It saved the changes and sent it to Flickr. Not counting the initial setup, it literally took 2 minutes from start to finish, and would have gone faster if I hadn't tweaked the circles a few times. It will also allow you to mark and annotate existing photos, or grab a pic from your computer's camera.
If you have a Mac, do any amount of work with screenshots or marking up photos, and don't have this already, get it. It rocks. 🙂