While the initial exercises are a bit shallow, they may catch the student's intrest and lead them to the deeper links. The Face of Russia is an impressive, interactive history of the country, based in part on the PBS series. Also includes iconographic icons. It's uneven, young, enthusiastic, and international. YaHooligans and Searchopolis are always popular.
Annotations are extensive and enthusiastic.
The Awesome Library for Teachers, Students, and Parents covers 14,000 resources. It offers different interfaces for kids, teachers, teens, and parents. Each bi-weekly newsletter focuses on a specific topic. Kids Web , from the Northeast Parallel Architectures Center at Syracuse University, is a directory of resources for K-12. Annotations are brief, but the subjects include the sciences, arts, social studies.