"The Creative Mind: Myths and Mechanisms"
If you want to delve into the cognitive side of creativity, read this 1991 monograph by Margaret A. Boden of
the School of Cognitive and Computing Sciences at England's University of Sussex
This comprehensive site offers "Resources for Creativity and Innovation"
A Swedish take on the topic and lots of links and thinks.
The Official Ed De Bono site
The Horse's Mouth.
Creative Center for the Universe
Right Brain Workouts - 87 aerobic exercises for the creative side of your brain.
Games and riddles to bend your mind. Ready, stretch.