There are many reasons for wanting to rip a DVD onto your hard drive. For example, if you like to watch movies on your laptop while traveling, you can save battery power by watching via a QuickTime file rather than using your DVD drive.
HandBrake does a great job of ripping those DVDs into MPEG-4 format. Instant HandBrake will let you create iPod-compatible MPEG-4 or H.264 files.
Check it out: HandBrake