Other
Teoria: Key Signatures
This site from Teoria's Books offers a short, easy to read article on Key Signatures. An interactive keyboard makes the learning more fun.
Other
Music Theory First Aid for Struggling Students
This site provides music theory information on such topics as rhythm, intervals, scales, chords, etc. Also, there is a fine section which describes the basic concepts of form and structure.
Music Theory
Music theory.net
An online beginning music theory and ear training site. This site is for students to use at home to reinforce what they learn in class.
Other
Dr. Brian Blood: Music Theory and History Online
An exhaustive resource with everything concerning music theory. From ear training to music history, this site covers it all.
Other
Music Fundamentals
This is like a music theory textbook online. It includes great explanations and assignments, although there is not a way to check your completed assignments. It begins with the basics and goes through triads. Midi files are included.