This site is funded by the National Science Foundation. It originates with the math faculty at Swarthmore College. It includes a site for students, teachers, parents and a research division. There is a math chat room for the kids as well as resources for parents and teachers. It seems that this would be an appropriate site for upper elementary and older.
Every day there is a new math challenge and opportunity to send in the solution.
Past challenges are also available at this site. This site includes games, puzzles, graphs and charts involving a wide variety of math skills. It is put out by the Los Alamos National Laboratory. This is a web site for music educators produced by the Music Educators National Conference. It provides resources that include music, music related news, industry contracts, books and publications, and connections to educators.