The following websites provide great practice opportunities for middle school students...
http://www.ixl.com/math/
http://www.coolmath-games.com/
http://www.funbrain.com/
http://www.aplusmath.com/games/index.html
http://www.mathplayground.com/
http://mathforum.org/
http://www.aaamath.com/
http://www.figurethis.org/index.html
http://www.mathcats.com/
http://www.mathleague.com/
http://www.ixl.com/math/
http://www.coolmath-games.com/
http://www.funbrain.com/
http://www.aplusmath.com/games/index.html
http://www.mathplayground.com/
http://mathforum.org/
http://www.aaamath.com/
http://www.figurethis.org/index.html
http://www.mathcats.com/
http://www.mathleague.com/