It is important to closely supervise children when they are online and talk about safety online. Some of these websites have pop-up advertising and may be a bit confusing for children. An adult should look at the website with the child before the child uses it. 

http://www.nzmaths.co.nz
This website is used by many teadhers to find resorces that allign with the way we teach mathematics in New Zealand Schools (not a games website)

http://arb.nzcer.org.nz/supportmaterials/maths/concept_map_basic_facts.php
This website does a great job of explaining different basic facts strategies and number terminology (not a game website)

http://www.tutpup.com/
A good website for fun activities that support numeracy

http://www.mathplayground.com/
A good website for supporting strand and numeracy

http://www.coolmath4kids.com/
More free online math games and other stuff
 
http://www.ixl.com/
This website has questions presented in a test type format

http://www.kidsnumbers.com/
Old school games with a math component

http://www.primarygames.com/math/numbereaters/start.htm
Some fun seed games but there is lots of advertising on this website

http://users.tellurian.com/teach/math.html
A variety of basic facts and other maths games

http://www.4kids.org/games/
A mix of literacy and maths games

http:/www.superteacherworksheets.com/
printable worksheets