These utilities allow you to manipulate QuickTime movies (and other QuickTime readable media such as MPEGs, AVIs, music and images, and others with appropriate plug-ins such as Ogg and WMV files) via the command line.
The QTCoffee tools allow you to modify movies by extracting tracks and chapters, creating chapters, scaling & rotating tracks (or the entire movie). You can also multiplex movies (e.g. take a video track from one movie and a sound track from another), join movies together, and split a movie into equal duration parts, at specified times, or by chapters. Files with only MPEG-4 video and/or AAC audio can also be saved as MP4 movies without conversion.
Why would you want such a thing? Well, if you only happen to need these particular features, and don't want to pay $30 for Quicktime Pro, then this is for you! More importantly, these utilities can be used inside shell scripts and are therefore very useful for batch processing. We think of them more as complements to Quicktime Pro rather than replacements for it.
QTCoffee is donation-ware for personal and academic use: if the tools are useful to you you are encouraged to purchase a license or pay whatever amount of money you feel is appropriate. If you use it for commercial purposes (even if you don't make any money specifically off the project) you will need to purchase a license at $10 per seat.
If you encounter bugs, please tell us! Reporting them in the ratings section below does not help.