Useful, free games to help kids remember addition/multiplication with regrouping, order of operations, negative and positive numbers and word problems. They also have some games that are available to play for a licensing fee.
MathCafe has online flashcards with immediate feedback.
Update: I found another set of games at Math Playground this morning. The kids haven't played any of these yet, but I have tried Math at the Mall. These look like good games for older elementary students.