MAC OS X - screencast
If you really can't spend any money, take a look at ScreenCastle. It's a web-based (uses Java) tool that works cross platform.
That said, on a mac, some of the greatest screencasts I have seen have been made with ScreenFlow ($99). It is very easy to use, has built in editing ability, and has great high end features. But it is not free.
I have been impressed with Jing. It allows you to take screencasts and screenshots and share them easily.
Another reason to hold off on investing in a screen recording tool right now, OS X 10.6 (Snow Leopard) has one built-in: Screen Recording in Snow Leopard's Quicktime X