This site has explanations of math topics, games, etc.
http://www.aaamath.com/

Many math sites are listed here.
http://www.kidsites.com/sites-edu/math.htm

Lecture notes, links, homework help and more are available at this site.
http://www.mathematicshelpcentral.com/

Both students and teachers can find help here.
http://mathforum.org/library/

Search for lesson plans, math topics and software.
http://archives.math.utk.edu/k12.html

If you find a good math site to share, let me know and I will add a link.

An interesting book to read about math instruction is entitled: The Teaching Gap by James W Stigler and James Hiebert. It compares math classes in Japan, Germany and the United States.