The MacTutor History of Mathematics Web Archive at the University of St Andrews is one of the most accessed resources worldwide for mathematics and its history. The archive includes detailed biographies of 2740 mathematicians and over 2000 other pages of essays on specific topics and supporting material, presented in a readily searchable form which engages and informs. It has had great influence on popularising and communicating the essence and importance of mathematics, inspiring a broad audience across the world, as well as being a vast educational resource. The site has sustained an average of two million hits per week over the last six years. It has been the basis for college courses worldwide and numerous student and school projects on mathematics and its history, and it has served as a seminal resource for many popular science, reference, and academic books, TV and radio broadcasts and lectures. The Archive continues to grow, with new material continually being researched and added.