Wonderville is a great, free site I just learned about from an article in Tech & Learning magazine. Wonderville is an award winning site that has interactive science activities that are fun and educational. There are over 30 games on Wonderville that help students learn about science while having fun. There are also puzzles, activity sheets and crosswords, videos exploring real-life applications of science and behind the scenes looks at science careers. The site even has some fun comics that entertain while teaching science facts.
The materials are very good, available for a wide range of grade and ability levels and cover many areas of science. There are also pages for students, families, and teachers to help you use the site and materials.