CamStudio. I've used it, very easy to use and lightweight. It's open source meaning you can change it if you know how to tweak the source code.
BB Flashback. It's a nice recorder, but requires a little skill to get used to the control. It exports things into FLV (Flash video) if you set it to.
Jing. I've never used it but it's by TechSmith which makes Camtasia Studio and I have that one. Jing is the freeware version of Camtasia Studio.
VirtulaDub. Never used it but reviews are positive.