There is a lot of material available; this is just a selection of some things I've come across.
Understanding Temperaments is a general introduction, with audio examples and a Java applet which lets you experiment with different tunings.
Kyle Gann's An
Introduction to Historical Tunings,
written by a composer
working with alternative tunings.
Audio examples of different tunings, from the Huygens-Fokker Foundation and from Wim Kamp.
Just Intonation Network is the home page of an organisation dedicated to just tunings and alternative scales. There is a lot of information here.
Pythagorean Tuning... is a long article on the history of tunings, particularly in the middle ages.
A spreadsheet for calculating temperaments.
Zarlino, a site (in French) about just intonation in Renaissance vocal music, with many audio examples.
Tuning and Temperament bibliography from the Huyghens-Fokker Foundation.
Music translated into Mathematics: Leonhard Euler
Maintained by d.griffel@bris.ac.uk