Below are some popular websites that feature explanations of various mathematical topics, practice problems and fun, challenging games.
http://www.mathforum.com/
http://www.aaamath.com/
http://www.coolmath.com/
http://www.figurethis.org/
http://www.mathcats.com/
http://www.easymaths.org/
http://www.mathleague.com/
http://www.funbrain.com/numbers.html
http://www.mathforum.com/
http://www.aaamath.com/
http://www.coolmath.com/
http://www.figurethis.org/
http://www.mathcats.com/
http://www.easymaths.org/
http://www.mathleague.com/
http://www.funbrain.com/numbers.html