Educational interactive websites are great for computer centers or for students during free time. At home, majority of children spend most of their time on the computer anyways, so why not expose them to fun educational websites?
@anacristinaprts posted a website created from Jefferson City Schools that has interactive websites categorized by subject. After each website's description, it gives the appropriate grade levels. It'll take some browsing to find what you might like but it's definitely a great resource!