Cam Studio records - but the trouble is, it doesn't edit. So the advantage of Camtasia is that you can zoom, add effects, transitions - fade in, etc - slow motion, etc etc. With Cam studio you are very limited.
Strangely, Camtasia is quite a lot cheaper on the Mac. So if you happen to have an (expensive) Mac, at least you'll get Camtasia cheaper.
The other free screen recorder is Debut. Then there's Jing, limited to 5 minutes.
Windows movie maker has a few, very limited, editing tools.
Malc