Please note: I am no expert on screencasting. The information below is information I have found online coupled with programs recommended by people who use these for various purposes.

Things to Keep in Mind


cost
real time capture/rendering
compression/file size
ease of use

Programs to Consider
- more details coming

Mac
iShowU - my school's program of choice
We are not using iShowU HD. The classic version (v1) @ $20 works well on both OSX 10.5 and 10.6 in our experinece.
Quicktime on Snow Leopard - Preloaded with OSX 10.6, ...
Screenflow - free trial, buy for $99
Snapz Pro X - rendering time is an issue because it occurs after you are done filming, file sizes are quite large

Windows

Camstudio - free
Fraps - free
Game cam
Ye

Both Mac and PC
Camtasia Studio
Jing - free, allows selection of part of your screen

Web-based
Screenjelly - free, records your whole screen
Screenr - free, allows selection of part of your screen
Screentoasters - free, offers choice of recording whole screen or selecting part of your screen via adjustable rectangle