Homework Spot. This is a good place to start. Everything from "why bother" to specific experiments, organized by grade level.
SciencePage. Murray Hart, lists sites that help you select, perform and report your project in an A+ way.
The Mad Scientist at work. Need we say more?
The Fun Science Gallery has instructions for many typical projects.
At The Cyber-Fair, you'll find descriptions of each step in the process, and help with the mechanics of creating your display board
Check out the Discovery Channel's Science Fair Central.
Reeko's Mad Scientist Lab. K-12 educational and fun science experiments for kids, children, teachers, and parents.
Scifair.org, sponsored by the Society for Amateur Scientists, walks you through the whole process.
Want to make that volcano explode? Find instructions for various kinds at Volcano World.
EdWeb has advice on every phase of your project, including lots of samples from a 7th grade class.
See what other kids are doing in the E-Journal of the National Student Research Center.
Arts Magnet School Science Fair Tips focuses on ccientific method, ideas, final display
Visit Science Fair Prep for preparation tips and ideas by grade level
Science Fair Handbook includes keys to a successful project and ideas by grade level
All Science Fair Projects has detailed projects with grade level difficulty