Investigations is a complete K-5 mathematics curriculum, developed at TERC in Cambridge, Massachusetts. It is designed to help all children understand the fundamental ideas of number and arithmetic, geometry, data, measurement and early algebra.
The NCTM site has lessons and resources available for members
A fractals unit for elementary and middle school students
The Mathematics Archives WWW Server has a list of many teaching materials pertaining to K-12 such as lesson plans, schools, software and useful links.
You can post one of your own lessons or look at many archived lessons from your colleagues.
Agnes Scott College maintains this site of biographies of women in mathematics.
Students can go to this site and have most of their math questions answered
Kathy Schrock provides excellent resources in all subject areas. This link is to her math page.
A searchable library covering most mathmatics topics
Hundreds of Math videos available for viewing and downloading
This site costs $45 per year, but has lesson plans, downloadables, and slide shows available.