This website tests students on basic arithmetic to see if they are the math mathematician. I enjoy this website because it offers varying levels of difficulty.
PBS kids is just great. This website URL will take you to the games section for kids. It does require a user name and password however instead of instant use.
This website I used directly in my instruction of fractions. It allowed me to have the students interact with the SmartBoard as well as give me some great "lecture"esque material to use.
This website is not just helpful for math but can be used for other topics. I used the Formula Fusion as I worked to find interactive games for my students involving fraction work. I really enjoyed this website!