Wolfram Research
Wolfram Math World: Four Color Theorem
A great discussion of the Four Color Theorem and its history.
Other
Georgia Tech: The Four Color Theorem
An explanation and proof of the Four Color Map Theorem. Includes illustrative images and a plethora of information.
University of St. Andrews (UK)
University of St. Andrews: St. Andrews: History of the Four Color Theorem
The University of St. Andrews offers a brief history of the first math problem to be solved with computers.
Other
Mathematical the Four Color Theorem and Computer Proof
This site describes the role of computer involvement in the four color map theorem.