Dent du Midi is an application that takes standard MIDI files and generates separate files for each track which contains the MIDI note data. These files are suitable for dropping into Apple's GarageBand application for use as loops or entire tracks. In simple terms, this application allows you to convert MIDI files into GarageBand files.
The name is a bit of a play on words. Dent du Midi is a 10,686 foot (3257 meter) mountain in the southwest of Switzerland in the west Alps. The name is French and is pronounced something like "don doo midi" which I thought sounded like "don't do MIDI" which is why it seemed to be an appropriate name since GarageBand doesn't import MIDI files.
Note that as of January 2005, the ability to import MIDI files has been added in GarageBand 2.0, so the need for Dent du Midi is only for those continuing to use GarageBand 1.x, for those looking for ways to translate MIDI files originally intended for MIDI devices that do not comply with the General MIDI standard for instruments and drum patches, and for those looking to split MIDI files into separate track files.