The Mac has a great set of keystrokes that will save screen captures (or portions of the screen) as fast as you can hit the keys. SnagIT can do all that, plus add graphical annotation, do simple video capture, and automatically upload your images. An excellent use of SnagIT is to capture an entire Web page (it will scroll the page down for you if necessary) and save it for documentation. SnagIT and Camtasia Studio make a powerful combo, and if you buy them together you can save some bucks on the purchase.
Compatibility: Mac OS X 10.6.8 or later