http://www.aaamath.com/add.html Addition from AAAMath. INTERACTIVE This site covers numerous aspects of addition, by explanation and interactive problems.
http://www.coolmath4kids.com/addition/index.html Addition from CoolMath4Kids (2-6) INTERACTIVE
http://www.edbydesign.com/automath.html AutoMaths Learning math should be a natural part of life, given that so many things involve the use of math facts. However, for some kids (and adults), math is not easy. It can be difficult to understand the logic behind math operations and to calculate these mentally. We've designed this math activity to provide a gentle and positive environment where children can practice their number skills in addition, multiplication, subtraction and division.
http://www.edu4kids.com/index.php?page=12 Education 4 Kids. (K-6) This site has online flash cards for addition, subtraction, multiplication, and division.