Sparked by an interesting Hacker News thread recently – if you need to implement blend modes or understand technically what is going on then you'll find these useful.
Photshop Blend Modes Explained
http://photoblogstop.com/photoshop/photoshop-blend-modes-explained
Insight into Photoshop 7.0 Blending Modes
http://dunnbypaul.net/blends/
Let’s Learn Math: Photoshop Blend Modes
http://www.venture-ware.com/kevin/coding/lets-learn-math-photoshop-blend-modes/
Photoshop Blend Mode Math
http://inlandstudios.com/en/?p=851
Implementations
Obligatory Wikipedia Link
http://en.wikipedia.org/wiki/Blend_modes