Mostly these apps will output AVI, so you might need the DivX codec to work with the files in quicktime (free) http://www.divx.com/divx/mac/
If the clips you are ripping off DVD are longer than 9-10 minutes, you'll need to upgrade Quicktime to Quicktime pro ($29.99) so you can cut them. http://www.apple.com/quicktime/buy/
There is other open source (free) video editing software, but I don't much about them.