Musictheory.net
An interactive website with music theory tutorials for students and
teachers by Ricci Adams. Lesson topics include: staff, clef, and ledger
lines; note duration; measure and time signatures; rest duration; dots
and ties; simple and compound meter; odd meter; steps and accidentals
and more. A chord calculator, staff paper generator, and matrix
generator are available for online use. You can download the stand-alone
version of the trainers and utilities. There is also a forum for sharing
information and asking questions. Nicely done, Flash intensive.

San Francisco Symphony Orchestra

This is a fantastic site with all orchestral instruments depicted and explained. Wonderfully it includes sound files of all the instruments so that students can listen to them as well. It deals with harmony pitch, symbols, rhythm tempo etc, and allows students to fiddle online.