There's a bunch of them on the Mac and probably a ton more for Windows. If you want to convert DVDs for the iPhone/iPad, at least if you have a Mac, then I highly recommend Handbrake. It's also available for windows, but at least a while ago apparently the Windows version wasn't nearly as good. However that very well may have changed by now, so it would still be worth trying. It's free.
I can't offer advice on programs that convert different video formats to iPhone/iPad as I've never done that much, but certainly there's a lot of programs that do that.