Scholastic Stacks has games based on popular books. Try a few.
Fact Monster has, surprise, facts. Enjoy. ( This site has ads.)
Math games, memory games, word finds, and more. (There are ads on this site.)
Light up your Brain
Brainteasers and audio stories for k-8.
Games for every subject, many designed for middle school students.
Learning Games for Kids
Games, lessons, videos, and activities for students kindergarten through high school. Challenge yourself.
Read Write Think
This is a long list of many different activities, for different ages.