Game Classroom (http://www.gameclassroom.com/)

From days-of-the-week questions for kindergarteners to geometry problems for sixth-graders, GAME CLASSROOM offers fun, informative ways to build on what kids are learning in school.The site's organization makes it easy to find age-appropriate activities. Parents will also find creative ideas about how to teach subjects to kids in the site's homework help sections.

Although some of the games are from other sites, kids can play them without ever leaving Game Classroom because the games appear within its browser window. While that ups the safety factor (kids shouldn't accidentally stumble across any objectionable sites, and the games come from kid-friendly sites like PBS Kids) they'll still see any ads that appear on the game site homepages.