A handy listing of various instrument transpositions.
Another useful site with plenty of information about music theory.
Music Theory for Musicians and Normal People
A website by Toby Rush (Professor of Music Theory and Aural Skills at the University of Dayton) with lots of fun, informational materials about many different topics in music theory and history.
A online dictionary of musical terminology.
A great resources that includes mini-lessons on many of the topics covered in class as well as aural skills training and music theory quizzes to keep your ears, eyes and mind sharp!