Timothy Gower, a British mathematician and winner of the Field Medal, likes to work collaboratively with his readers.
Leonhard Euler Links to sites with information about Leonhard Euler and various formulae. The Euler Formula at Wolfram's MathWorld. Proofs of the polyhedra ratio from The Geometry Junkyard and The Ohio State University. Biographical information at University of St. Andrews.
Some of the many blogs that are being produced by mathematicians. I'm always looking for new and interesting sites. Submit suggestions above, and I might include them here!
These sites, created by societies and researchers, cover a variety of mathematics topics.