Timed Screenshot is a tool that allows you to take continuously timed screenshots of the desktop and specified intervals. It can be used to monitor what is happening on a remote machine.
When you launch this application it runs in the background till you kill it on the Terminal or you log out. There is a configuration file (plain text) where you can change some settings like the time interval in seconds, the name of the destination folder (which can be an alias) and the quality of the JPEG compressor in percent.
The files are named e.g. "2003-06-05-21-42-48-0.jpg". Year, month, day, hour, minute, second and ID of the screen. It should work with multiple screens, but it's not tested.
The application will quit when you shut down your Mac or you log out. To quit at any time you may need the force quit dialog.