A member of the Marco Polo set of partners, this website, sponsored by the National Council of Teachers of Mathematics (NCTM), offers many free lessons, activities, standards alignment, and other information.
With 102 online activities and 515 lesson plans for grades preK-12, it'll be a long time before you run out of information on this NCTM-hosted site for mathematics. All resources are sorted by grade level and include online interactive games to teach subjects like fractions and probability, as well as activities that can be done using pen and paper or crafts materials and a bit of ingenuity. The site also includes school math standards and links to other useful web resources.
Published by National Council of Teachers of Mathematics in 2000