A few of my favorites:
http://illuminations.nctm.org/Lessons.aspx
http://www.homeworkspot.com/elementary/math/
A+ Math Flashcard Creator (create flashcards at home to practice your skills)
http://www.aplusmath.com/Flashcards/Flashcard_Creator.html
Math Facts Interactive Game
http://www.playkidsgames.com/games/mathfact/mathFact.htm
Addition Attack (add single and double digit numbers while trying to save earth)
http://funschool.kaboose.com/formula-fusion/games/game_addition_attack.html
Math Baseball (kids can practice addition, subtraction, multiplication, and division as they play Math Baseball -- where every correct answer scores a homerun)
http://www.funbrain.com/math/index.html
Two Minute Warning (race against the clock to see how many problems you can answer in two minutes)
http://www.primarygames.com/flashcards/multiplication/start.htm
Geometry and Measurement
http://nlvm.usu.edu/en/nav/category_g_2_t_4.html
http://ejad.best.vwh.net/java/patterns/patterns_j.shtml
Fractions and Decimals
http://nlvm.usu.edu/en/nav/frames_asid_104_g_2_t_1.html
Math Dictionary
http://www.amathsdictionaryforkids.com/