When you rip a DVD to your Mac you end up with a VIDEO_TS folder, which can be dragged to DVD player icon to play the DVD (you might find some stubborn DVDs will only play in VLC though).

As the DVD media is a folder, obviously you can't double-click it to open it in DVD Player. Instead you have to do the cumbersome open-the-app-then-drag-and-drop-to-icon procedure.

The way around this is to rename the folder with the .dvdmedia extension. This will turn the folder into a bundle, and add a nice DVD icon as well. Double-clicking this will automatically open it up in DVD player, but you can easily change this to another app like VLC for example. The easiest way is to choose Get Info on the bundle, choose the new app from the "Open with" section, and click the "Change All..." button. While you are in the Get Info window you might also want to click “Hide Extension” and rename the bundle to the name of the movie. You can also change the icon to the artwork for the movie which you can get from a site like Get Video Artwork.

One annoying side effect of this is that Front Row won't recognise dvdmedia bundles, whereas it will recognise VIDEO_TS folders. Of course, to get your movies to work well in the Finder, Quicktime iTunes, Front Row and iPhone all at the same time you might prefer to run them through a tool like HandBrake. This can also save you some space depending on how you encode them.