Math portal
from 610 portal
Greg Tang Math http://gregtangmath.com – Kakooma is a particularly fun and fast-paced game on this site
NCTM Illuminations http://illuminations.nctm.org/ActivitySearch.aspx
Bedtime Math http://bedtimemath.org – there is also a Bedtime Math book available!
Thinking Blocks (mathplayground.com)