Khan Academy--Literally thousands of instructional videos
Interactive Simulations--Perfect activities to use on a SMART Board!
Math Blog--Math teacher, turned researcher, Dan Meyer's math education blog.
National Library of Virtual Manipulatives--More math simulations and manipulatives!
Math Tools for Kids!--Links to dozens of K-12 math sites from Trumbull County, OH Educational Service Center.
Illuminations--This is a great site built by the NCTM.
Dyscalculia.org--A very comprehensive site of math tools and research addressing math learning disabilities; some free, some for pay.
A Maths Dictionary for Kids--This site comes from a teacher in Australia. Very interactive!
Squidoo Help with Math--More great sites listed, including a few above.