Eisenhower National Clearinghouse for Math and Science Teachers (Ohio State University) http://www.enc.org/ This umbrella site is a gold mine for mathematics and math education. You can read newsletters, specialized catalogs, and topical publications, or connect to other education sites. An Online Documents service allows you to find articles on curriculum issues in mathematics that are available in an electronic format.
A Lessons and Activities page includes classroom activities with projects and supporting materials. When we last checked, the page contained sixteen different activities; the following three titles give a flavor for the kind of things that can be found here.
Mathematics Forum (Swarthmore College)
This is a new site, extending the well-known Geometry Forum, which was developed several years ago at Swarthmore. The site is intended to create an on-line mathematics community; it hosts discussion groups on teaching mathematics, provides useful information and interesting math problems, and has a search engine for mathematics resources.
One of the useful features at this site is Ask Dr. Math. K-12 students can get answers to their mathematical questions by linking to the Dr. Math Archive, which is a database of previously and frequently asked questions in mathematics. Students can also send a message directly to Dr. Math (actually a team of mathematics students at Swarthmore) using the email address firstname.lastname@example.org. Replies are usually sent within 24 hours.
NCTM Standards (National Council of Teachers of Mathematics)
A complete copy of the NCTM Curriculum and Evaluation Standards for School Mathematics can be found here, which includes a standard for discrete mathematics in grades 9-12. This excellent resource includes a discussion of each standard at its designated grade level groups: K-4, 5-8, or 9-12. These discussions contain a wealth of ideas and materials that could be adapted for use in the classroom. This is a great starting place for anyone who is interested in mathematics education.