Based on the source code of GIMP and a bit of reverse engineering from Photoshop's behaviour (there were inconsistencies).
This is used in http://colorblendy.com/.
Unfortunately this doesn’t (1) show you how the blend modes actually work in practice, or (2) tell you what to do with each blend mode or why you should care about it.
The text is okay, but text really isn’t a good medium for quickly/concisely conveying information about visual effects. (I’m reminded of mathematics papers from the days before we had mathematical notation.)
I really like this resource, from 2006: http://dunnbypaul.net/blends/
Koeri in these comments recommends Kai’s Power Tips. Also good is the book Photoshop Channel Chops, from the mid 1990s, now out of print. Much more useful than almost all of the Photoshop books published more recently.