University of Michigan
U.michigan: Keyboard Temperament Analyzer
Features a downloadable spreadsheet for analyzing keyboard temperaments. Requires a bit of physics knowledge.
OpenStax
Open Stax: Catherine Schmidt Jones: Tuning Systems
Here is a good introduction to the various tuning systems. Topics covered are Tuning Based on the Harmonic Series, Pythagorean Intonation, Mean-tone System, Just Intonation and more. This is also available as a printable pdf file.