Arcademics A website where kids practice math facts and concepts through arcade games.
Illuminations This is a fabulous website by the National Council of Teachers of Mathematics that has lessons and interactive games.
Johnnie's Math Page A website that divides the concepts by grade and then by topic. It has a lot of games to practice math facts and concepts.
Math Cats A website where students figure out logic problems.
Math Playground This website has a plethora of games all arranged by topic and grade.
Number Bonds Fractions This is a game where children can practice creating number bonds. There is a learning curve to this game, but it can become addictive quickly.