• Famous Problems in the History of Mathematics
• The Hilbert Problems 23 problems posed by David Hilbert at the turn of the century (1900).
• Hilbert's Problems another reference.
• Interactive Euclid's Elements
• International Math Olympiad Problems.
• Math Books of General Interest.
• Archive of Putnam Examination problems.
• More problems 3600 Olympiad and 900 other problems. Extensive list of Putnam solutions
• Teaching with Original Sources in Mathematics a course at New Mexico State University
• Higher Level Maths (filled with neat applets!)
• List of References on Mathematical Counterexamples

The Mathematical Tourist, by Ivars Peterson. W. H. Freeman and Company, 1988. ISBN 0-7167-1953-3

Mathematical Thought from Ancient to Modern Times, by Morris Kline. Oxford University Press, 1972.