I use Roxio Crunch to covert them on the Mac. It does a great job. However, you'll only be able to covert DVD's that do not have encryption protection. If you want to "crunch" a commercial DVD, you'll need to rip it to remove the protection. For that you can use Mac The Ripper.
Some of the copy protection on the newer DVD's makes it tough to do this. Talladega Nights is one that I tried and could not get to work. After ripping it to my hard drive, I get an error when trying to convert the Video, saying it has "bad sectors".
Anyway, Roxio Crunch is very slick and easy to use. (Like most things Mac) I am sure there are free alternatives out there if you want to scour the web, but for the cost of an average iPhone case, I'd recommend the purchase of this one.