a great collection of games in conjunction with many popular books, most are for young children but a few will appeal to older kids too also on the left hand side of the page you'll see links for teacher resources
MathSphere offers free resources for children age 5-11
TopicBox is a free directory of over 2,000 educational websites, categorised into subjects
Awesome Library is an American resources list site. Everything is very neatly divided into catagories which should help speed up searches for new sites of your choosing
Sites for Parents is a huge list of resource links
Sites for Teachers is a huge list of resource links
Whilst not very visually appealing, clicking on a section will take you to another visually tortuous but helpful list of resources for that given topic