The very first web site reviewed on MATHNEXUS was The MathForum. And rightly so, as this web site has helped a great number of mathematics teachers and students over the past years.
It seems appropriate to keep revisiting and re-suggesting the web site, to keep it in the minds of mathematics teachers as a great resource. Thus, let me recommend a special part of The MathForum's web site, known as MathTools.
First, MathTools provides a catalog with links to every grade level or subject area a mathematics teacher might teach....from pre-Kindergarten to Calculus. Then, once you access a particular grade-level or content-area link, you obtain access to a "ton" of links to related content. For example, within the Geometry section, one can find links to content about the history of geometry, coordinates, angles, line properties, polygons, circles, proof, congruence and similarity, measurements, transformations, constructions, and special topics (e.g. Fibonacci sequence, fractals, spherical geometry, hyperbolic geometry, or geometric probability).
And, once you click on one of these sub-topics, you obtain a detailed list of additional content. For example, within the area of Transformational Geometry, you can access a wide range of activities (most are technology-based), lesson plans, ancillary materials, or tools.
In short, The MathForum's MathTools should become your ever-present resource to help in planning lessons, extending lessons, enriching lessons, and self-motivated professional development. I offer this guarantee: You will find something of value every time you visit the site and explore its wealth of content and teaching ideas. Yes, MathTools Rules!!