Important Note: This app is no longer essential if you have OS X 10.6+ and Apple Quicktime X as it now has the ability to capture video from your desktop. If you have an older version of OS X however, it's worth downloading.
DesktopRecorder is a free desktop recorder for Macs which records both images and sounds in a simple handy app.
All you need to do is select the compression rate and number of frames per second you want your screencast to be in and you're ready to go. It's a very simple piece of desktop recording software but as noted above, so is Quicktime X which does exactly the same job and offers slightly more power than DesktopRecorder.
If you don't have OS X 10.6+ and QuickTime X installed, then DesktopRecorder is a very useful screen recording tool.