If you're looking to backup your DVD collection digitally (because we all know pressed DVDs don't last as long as DVD-Rs - and we want to watch these movies in 100 years) or format shift one of your DVDs onto an iPod or PSP, then I'd use Handbrake - make use of your Fair Use rights. If you 'own' it you HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded DVD to MPEG-4 converter, available for MacOS X, Linux and Windows.
[url]http://handbrake.fr/?article=details
Put your DVD in one end, and get a file out the other end.
Obviously if you're looking to copy a disk for your PC or iPod or something and it has something other than standard CSS encryption then you may need to look at 3rd party tools. Google is your friend. Doom9.org have a few links and resources.