This is a small collection of sites that I think are particularly worthwhile. You can link to them directly from the sidebar on the home page, but this page gives a brief description of each site, in addition to providing the URLs and links.
Core Plus Mathematics Project (CPMP)
CPMP is an NSF-funded project to develop a complete three-year high school mathematics course for all students, plus a fourth-year course continuing the preparation of students for college mathematics. The curriculum builds on the theme of mathematics as sense-making and includes the following major features: multiple connected strands, mathematical modeling, accessibility to all, use of graphing calculators, active learning, and multi-dimensional assessment.
This project consists of three series of year-long professional development programs for teachers of elementary, middle school, and high school mathematics. The programs are based on videotapes and use online learning communities for teachers to share ideas.
This site contains brief summaries of, and full reference information for, recent articles about mathematics that have been published in such periodicals as Discover, Nature, New Scientist, Science, and Scientific American.