How Stuff Works. Excellent site, arranged by topics. By Marshall Brain. (Book of same name.)
How Things Work. Answers to questions. By Louis Bloomfield. (Book of same name.)
Experiments using commons household items or your computer. Give a raspberry and watch your monitor respond!
Newton's Apple. Simple experiments to try, with short explanations. Meant for elementary students, but would be interesting to any age. From the TV show.
Statistics sites about many topics.
See Students for project ideas.
See Motion for the physics of sports.