Oldie but Goodie - EyeJot

Sometimes old (by Web 2.0 standards) is good. Originally introduced in 2007, Eyejot is a slick little applet that runs from a browser tool bar button; no software installation required. Just go to www.eyejot.com, register (it's free), login, and drag the provided link button to your browser's tool bar. On Firefox, it looks like this:

You can also find an EyeJot widget on their site, which can be installed on any site that will accept HTML. For several sites, including this one, Eyejot can be installed with a single click; look in the right sidebar and try it out (if you have a web cam).