Harcourt school: www.harcourtschool.com
Space Hopscotch: www.mathsyear2000.org/magnet/minus/space/index.html Counting by 2's. Followed by counting backwards.
AAA Math: www.aaamath.com Excellent explanations followed by activities. Well categorized into over a dozen topics. Divided into grades K-8.
A+ Math: www.aplusmath.com Matho (like Bingo), Concentration & Hidden Pictures. 
Flashcards and worksheets available.
Math Baseball: www.funbrain.com/math/index.html Choose addition, subtraction, multiplication, division, or all. Easy to "Super Brain." Algebra or non-algebra. Click the "Play Ball" button. Click in the blank box and type the answer. Easy problems are singles. Difficult problems are home runs. Three incorrect answers result in an out.
Fresh Baked Fractions: www.funbrain.com/fract/index.html Choose a level. Simplify the fractions and click on the one that is not equivalent.
Power Football: www.funbrain.com/football/index.html Choose addition, subtraction, multiplication or division problems with decimals. Select level of difficulty. Plays like football.
Place Value Puzzler & Rounding: www.funbrain.com/tens/index.html Choose a level of difficulty. Round problems or estimate.
Shape Surveyor: www.funbrain.com/poly/index.html Choose area, perimeter, or both and a level of difficulty. Calculate the area or perimeter of shapes and type answers. Correct answers reveal a hidden picture.