It's often a shock to many, but there are several professional societies for mathematicians. A few of the bigger ones are:
Eric Weisstein's World of Mathematics, hosted by Wolfram Research, the makers of Mathematica.
The On-Line Encyclopedia of Integer Sequences; give it an integer sequence, and you'll probably find out more about it than you ever wanted to know.
The Journal of Online Mathematics and its Applications; several interesting and reasonably understandable articles can be found here which were specifically designed for an electronic format (e.g. interactivity, etc.)
The Electronic Journal of Combinatorics, perhaps a bit steep for a general audience, it's nonetheless near and dear to my heart.
The BRIDGES directory, and annuals conference about "Mathematical connections in Art, Music and Science".
George Heart, a computer science professor at SUNY Stony Brooke and prominent "mathematical sculptor".
The Geometry Junkyard, maintained by Davied Eppstein at UC Irvine. Everyone needs to check this site out at least once.
The Great Internet Mersenne Prime Search, also known as "GIMPS" - a massive distributed computing venture which has discovered a handful of the largest known prime numbers. If you're so inclined, it's easy to help out by downloading their (free) program which runs in the background on your PC, using only the otherwise "idle" clock cycles. There's even prize money ($100,000) for discovering the first ten-million digit prime number.
Millennium Prize Problems; the Clay Mathematics Institute has put up a million dollars in prize money for the solution of each of seven long-standing open problems in mathematics.