http://tqd.advanced.org/2647/geometry/intro/intro.htm
This site is great for the basics of geometry. A teacher could use this as a reference or students could use this to learn concepts of geometry. The glossary is especially nice for high school age children.
http://www.ics.uci.edu/~eppstein/geom.html
This site gives more links to website pages related to geometry. As a teacher, you would need to find the pages that are appropriate for your classes. If you click on the Geometry Junkyard, you can open eppstein's collection of geometry topics--but even that is just another collection of links.
This site is a gold mine for a high school math teacher! There are lessons, newsletters, discussion groups, problems of the week. The Ask Dr. Math link allows anyone to pose a question, but beware, usually only a reference to the archives is given.
This new feature contains ready-to-use lesson plans, activities,
worksheets,and Web sites related to the Olympics. And,
just in time for the Games, we've added five super sections that
bring
you daily news updates from USA TODAY, inspiring athlete stories,
profiles of past host nations, and more. Your students can even
email questions to an Olympic reporter on-site in Sydney to get
the "inside scoop."
MATHEMATICAL MISTAKES
http://www.math.vanderbilt.edu/~schectex/commerrs/
This website is useful for teachers and students. This website analyzes common mathematical mistakes made by undergraduate students. Even High School students can benefit from reading this site to isolate their own common errors. Teachers can benefit by pointing out common errors to keep students from making these errors.
This is another website useful for teachers and students addressing common mathematical mistakes. These are the mistakes made by advertisers, the media, reporters, politians, activists, and in general, non-math people.The web page design is much more interesting and colorful, so it may be more attractive than others. This site is maintained by Paul Cox. It also includes puzzles & problems, mistake of the month, mistake archives, in the news, and sources and links.
MATHEMATICS OF THE PRESIDENTIAL ELECTION
http://hometown.psn.net/~xocxoc/math/election.htm
This is a website that was located as a link from another page. It details the mathematics of a tied election. There is great discussion of percentages, margin of error, statistical data from Florida votes, and possible election reform.