Khan Academy
http://www.khanacademy.org/

Illuminations
http://illuminations.nctm.org/ActivitySearch.aspx

New Zealand Math Numeracy Project
http://nzmaths.co.nz/

IXL
http://www.ixl.com/math/

Virtual Manipulatives
http://nlvm.usu.edu/en/nav/grade_g_1.html

Oswego Math Games
http://resources.oswego.org/games/

TVO Kids
http://www.tvokids.com/

First in Math
http://www.firstinmath.com/

Math Playground
http://www.mathplayground.com/

The Math Forum
http://mathforum.org/

AAA Math
http://www.aaamath.com/

Cool Math
http://www.coolmath.com/

Figure This
http://www.figurethis.org/index.html

Math Cats
http://www.figurethis.org/index.html

Hooda Math
http://www.hoodamath.com/

Fun Brain
http://www.funbrain.com/brain/MathBrain/MathBrain.html

Need help solving a math problem? Enter it at Math Way
http://www.mathway.com/

PBS Kids Cyber Chase
http://pbskids.org/cyberchase/math-games/

HSP Math
http://www.hbschool.com/hspmath/na/index.html

Erich's Puzzle Palace-Tons of criticle thinking games
http://www2.stetson.edu/~efriedma/puzzle.html

Math-O-Rama
http://www.eduplace.com/kids/mw/

ICT Games (UK aligned)
http://www.ictgames.com/resources.html