The National Museum of Mathematics:
http://momath.org
NYSED Common Core State Standards for Mathematics
http://www.p12.nysed.gov/ciai/common_core_standards/pdfdocs/ccssi_math_standards.pdf
Source for math teachers: Jefferson Math Project
http://www.jmap.org
College Board:
http://www.collegeboard.org
Math Forum at Drexel College (see the Problem of the Week or Ask Dr. Math):
http://mathforum.org
Summer Math and Science Programs:
http://syosseths.com/documents/Summer%20Science%20&%20Mathematics%20Research%20Programs%202011.pdf
Math for Teachers and Students – lessons and worksheets
http://www.aaamath.com
Mathematical Challenges for the entire family – National Council of Teachers of Mathematics (NCTM):
http://www.figurethis.org/index.html
Games that Instruct – NCTM
http://illuminations.nctm.org