http://www.fun4thebrain.com/addition.html --Practice addition, subtraction, multiplication, & division (great, fun games!).
http://www.multiplication.com/interactive_games.htm --LOTS of interactive multiplication games (really neat, colorful games)--Favorite!
http://www.arcademicskillbuilders.com/ -- Fan Favorite! This site includes the racing and tug-o-war games. Includes all 4 operations.
http://www.oswego.org/ocsd-web/games/Mathmagician/mathsmulti.html --timed flash cards (excellent for practicing speed!)
http://www.multiplication.com/interactive/quickflash/flash/index.html --timed flash cards (multiplication)
http://www.blackdog.net/games/math/flash/index.html --flash cards
http://www.blackdog.net/games/math/multiply/index.html --match problems with their products (multiplication)
http://www.classbrain.com/artgames/publish/multiplication_station_game.shtml --find missing factors
http://www.mhschool.com/math/2003/student/activity/courses/gr3/ch09/ --Matching Game (timed/not timed)
http://www.oswego.org/ocsd-web/games/SumSense/summulti.html --place factors and product in right order
http://www.oswego.org/ocsd-web/games/SpeedGrid/Multiplication/urikamultires.html --find missing factors
http://members.learningplanet.com/act/count/free.asp --practice skip-counting
http://www.learningplanet.com/sam/sm/index.asp --Space Game
http://www.aplusmath.com/Games/PlanetBlasterBasics/index.html --Planet Blaster Game
http://www.aplusmath.com --flashcards and games
http://www.quia.com/cc/66145.html --Memory Game
http://www.quia.com/mc/66145.html --Matching Game
http://www.coolmath-games.com/
http://www.kidsnumbers.com/long-division.php --long division practice
http://www.quia.com/rr/208064.html --long division: "Rags to Riches"
http://www.quia.com/mc/319832.html --long division: "Memory"
http://www.funbrain.com --Fun Brain
http://math-drills.com/ --free worksheets for drill/practice -- Excellent site!