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I tested a few before settling on iShowU.

Snapz didn't work very well for me. It recorded fine, but iMovie 08 did not like something about the files I created. When I shared them from iMovie, I got horrible results. Usually the audio playing, but the video being stuck on one screen.
 
I purchased iShowU a few months ago and love it. Works great, just be sure you set the right encoding if you want to upload to something like Youtube.
 
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