Amateur Scientist
http://www.eskimo.com/~billb/amasci.html
This site has lots of great information. Links to many sites. Some of particular interest are Kids' Science Projects Elsewhere and Physics Demo Page. Physics Demo Page includes details of experiments and links to other good sites for related information and research.

The Wonders of Physics
http://sprott.physics.wisc.edu/wop.htm

Chemistry Teaching Resources - Demonstrations and Experiments
http://www.anachem.umu.se/cgi-bin/pointer.exe?Demonstrations

Quick Demonstrations for Biology Classes
http://nesen.unl.edu/methods/biodemo.html

More Science Demos from the 'Net
http://nesen.unl.edu/methods/moredemos.html

Science is FUN
http://scifun.chem.wisc.edu/

Reduced Gravity Demonstartor
http://zeta.lerc.nasa.gov/mini/minit.htm