Are you talking about data DVDs or (commercial) movie DVDs?
Data DVDs can be burned for free using Disk Utility (make disk image -> burn that), cheaply with nice applications like
Disco or - if you want the best - spend some money on
Toast (which just has come in a brand new, universal version, and looks very good).
With movie DVDs you need to take another approach, you can either use something like MacTheRipper to - well - rip the TS_FOLDER, and then use Popcorn (or Toast) to burn that back to a blank DVD
(not entirely sure about the procedure, as I've never done that, others might provide corrections/additions), or if you don't need an exact copy, but just want to backup your DVD collection, use Handbrake to rip the DVD in H.264/AAC mov files that can be watched on any (decently spec'ed) computer with QuickTime or better (

).