The World's Money Exchange: This site will allow students to research the conversion of different countries' currency.
NCES Classroom: The National Center for Educational Statistics offers and excellent resource for educational data to enhance the math curriculum..
Illuminations: Based on the NCTM standards, this online resource offers rich, comprehensive math content in five grade level groups K-12. There are online math investigations, reflection activities with teacher video vignettes, reviewed web sources, Internet-based lessons matched to standards and searchable NCTM standards.
Swarthmore Math Site: This site has everything for math teachers and students. A favorite part is the Library of Problems of The Week with published, and quite different solutions, from around the USA.