http://homeworkspot.com/ This site is a secret weapon that serves as a hub for links to conquer all homework assignments, not just math. Some of the links are less educational than others.
http://www.funbrain.com/ A site for online educational games (math, grammar, science, spelling, history) recommended for students 3rd grade and younger. Has a very strong push to buy a membership. The free games are worth the visit however.