Jing is decent, and either free or dirt-cheap depending on which version you get.
http://www.jingproject.com/
The main advantage of the paid version ($14.95/year) is that it lets you save to MPEG-4 video as well as Flash SWF (much easier to edit using iMovie, upload to YouTube, etc.). Also the paid version doesn't slap a Jing logo on your video.
Just Windows and OS X, unfortunately -- no Linux version.
It's not as sophisticated as some of the others, but it's definitely usable for basic work. Maybe try the free version to see if it's good enough, then move on to the spendy stuff (Screenflow, Camtasia, etc.) if it's not.