Math Nexus - News and Ideas for teachers and learners of math. 

Cornell University Math and Science Gateway - This site has lesson and ideas for a multitude of math disciplines: Algebra, Geometry, Fractals, History of Mathematics, Tables, Constants, and Definitions, Funny, Irrational Pages about Pi and Other Numbers, Free Software and General Topics. Rich in resources!

National Science Digital Library The nation's online library for education and research in science, technology, engineering and mathmatics. 

Mathmistakes.info - This web site is provided to help high school and college/university students eliminate common mistakes and to learn important facts. Links to other resources as well.

Math Forum for High School staff -- lesson plans and ideas

Wolfram Math World -

MathWorld currently features a number of innovative interactive elements that enhance its usability for a variety of different readers. These features include: