General science
Games and puzzles to do with mathematics and science
Funbrain helps you learn the periodic table with a game
If you need help or inspiration for your science fair, try this site
The Exploratorium is described as a museum of science, art and human perception, and is part of the San Francisco's Palace of Fine Arts. Check out the activities.
Activities and games to do with the science of light, put out by the Optical Society
On the Science Hobbyist site, which is worth a look in its own right. The Science Club section has a variety of science projects, from simple to advanced, and links to other science sites.
Science activities from Australia's national science agency CSIRO
Physics
Good, fun explanations of various physical principles
Biology
interactive biology learning activities, such as cartoons explaining intestinal gas, and and an evolution lab