I have been going through this study/slide presentation that Susan Weinschenk created. Its got a lot of great info on the psychology of why people do certain things and how it should affect your web design and strategy.
I especially liked:
#1. You have Inattention Blindness
#10 You Want More Choices and Information Than You Can Actually Process
#33 Bite-Sized Chunks Of Info Are Best
#36 People are Inherently Lazy
#38 Even The Illusion Of Progress Is Motivating