Certainly there are many site on the web that try to explain physics material or concepts.  What I have compiled here are just a few of the ones that I've stumble across and think may be useful in the classroom.  Some sites are good examples of interactive HTML; others may be a nice supplement to the standard physics textbook. 

PAW: (Physics Around the World) is nice physics search engine. 

SciEd: a variety of links & info on all fields of science at the University of Washington. 

Frank Potter’s Science links: A simply amazing collection of links about anything and everything.  Not only sorted by subject, but also by target audience (there is information here for grade school kids to grad students). 

Conversion factors: when you need to know how many liters there are in a bushel. 

WebElements: one of the most complete periodic tables on the web, but it can be kinda slow. 

Physical Constants: a huge listing of the "official" values; thankfully it's sorted into searchable categories. 

Physical Constants: a smaller listing that usually downloads faster. 

The Chart.: should you be in need of basic HEP data, or just want to learn more about that damn poster that is in every physics hallway/ classroom. 

The Integrator: if you, or your students, doesn't feel like integrating by hand today. 

Physics 2000: Okay, it is a corny site name, but this may be one of the better examples of on-line physics teaching.  It’s a well produced site on modern physics with many Java applets.  

Feynman Online: learn about one this countries strangest characters who just happened to be a great physicist. 

Physics is For You: An exciting introduction to physics and your world (well, that's what they say). Actually, this is a cute site to send your students to on day one. 

Physics Humor: a nice site if you're looking for a laugh and want to waste some time.