On the Windows platform I was very fond of Jing (http://www.techsmith.com/jing.html), it is an easy to program which allows you to quickly take screenshoots or videoes, comment on them and share or save in a few steps. Very handy also at my work to document something to a client. And it is free!
On the Mac I came across “Skitch”
It seems that Skitch can do the same, so I have installed it and will write more later when I have had some more experience with using it. If you have some experience with this free app for Mac OS or have other suggestions to apps which can do as Jing on windows, please feel free to comment on this post.
Here are some more information for the Skitch app:
Download/read more: http://itunes.apple.com/us/app/skitch/id425955336?mt=12