I've been using Squared 5's MPEG Streamclip for Mac OS X for a while now, and it truly rocks. It has allowed me to do all kinds of things for Temple Faculty that were heretofore a real pain in the ass, mostly capturing clips from DVDs.

I'm not sure if it works with store-bought DVDs (like movies) because we've only used it for ripping DVDs people have made with iMovie/iDVD or similar products.

Here's the description:

MPEG Streamclip is an application that converts MPEG files (including transport streams) into muxed, demuxed, QuickTime or DV files with more than professional quality, so you can easily import them in Final Cut Pro, DVD Studio Pro and Toast 6. The most important conversions require the Apple MPEG-2 Playback Component (you can buy it online from Apple, but you already have it if you use either Final Cut Pro 4/HD or DVD Studio Pro); this component is used for high-quality decoding of MPEG-2 video. MPEG Streamclip also includes a player to set In and Out points, cut unwanted parts and perform a partial conversion.