There is an abundance of software to rip DVDs. I rent from Red Box then rip to hard drive on my PC or mac for later viewing. I only watch movies once and have no issues doing this. In fact I have not even watched half the movies I have ripped--nor would I have rented them had I not been able to rip for later viewing.
I refuse to pay the absurd prices for online rentals, such as from iTunes.
There are three legal issues here with this scenario. First, you didn't buy the DVD movie, so cannot claim the "fair use provision" in making a backup copy. Renting a movie does not give one license to make a separate copy. Doing so is a violation of U.S. intellectual copyright laws.
Second, ripping the DVD to a computer is a violation of the "space shifting" provision of current copyright laws.
And finally, virtually all movies available on Redbox utilize Digital Rights Management(DRM) or copy protection, namely CSS for DVDs and AACS for BluRays. Circumventing DRM by using ripping apps is a violation of the Digital Millennium Copyright Act(DMCA).
You can find a basic FAQ
here.