Flip4Mac recently announced the public beta of Drive-in, an application that allows you to create and store disk images of DVD movies for playback on your Mac (their FAQ indicates they may also be working on a Windows version -- no big surprise given that Flip4Mac is best known for developing Windows Media Components for QuickTime and thus enabling Mac users to view Windows Media files). Tools for capturing video from a DVD and storing it on your Mac are not new. The most well known such tool is probably Handbrake, which allows you to easily encode video for playback within iTunes (or for syncing to an iPod or Apple TV).
Drive-in is different in that it captures the entire contents of the DVD. This means that you have access to all the DVD menus and special features, including alternate audio tracks and commentary. This is because Drive-in is actually copying the entire DVD to a disk image file. That file can then be mounted and is treated by the Mac as if it were an actual DVD that you had inserted into the computer. To prevent piracy, Drive-in also ties each image file it creates to the registration code of the user who creates it (though that code can be used on multiple computers).
What I find most interesting about Drive-in is that it achieves the one goal that buying movies from the iTunes Store or using a tool like Handbrake to extract video from a DVD cannot -- providing access to commentary and other special features. Like most people, I love special features and I tend to check out which features are included on a DVD even before watching a movie. The downside is that Drive-in doesn't provide files that are particularly portable -- they cannot be loaded into iTunes and they can only be played on the computer (as opposed to an iPod, iPhone, or Apple TV).
If you've created a home media center using a Mac, this is great because it backs up your DVD library and it provide access to all the features of a DVD. Another potential use is a Mac laptop because when you're traveling, you might want to have the full content of several DVDs with you but without having to carry them. That said, many people tend to only listen to commentary on a DVD or watch special features once -- but may watch the movie itself many times. For those people, tools that extract the video and store it in a compressed file format (which can be much smaller than an entire DVD image) might be a better choice -- particularly if you want to be able to sync it with another device.