A few fixes for the music theory junkies...

Rudimentary Music Theory
Free text in music theory and orchestration
Music Theory Net - from basic theory to advanced chord analysis
A complete free course - from Murray State University
Treasure Trove of Music - quick reference
Music Notes

More Advanced
Music Theory Resources
Tonality and Harmony
Guide to Contextual Listening
Intro to Species Counterpoint
Harmony.or.uk - advanced study of chord progression
The Tonal Center - chords, scales, cadences and modulation

Eartraining & Interactive Theory
Big Ears - the original
Teoria.com - excellent flash interactive exercises
MeGill's Applets - great collection of exercises

Really Really Out There
Society for Music Theory
Eunomious

A Few Music History Resources
Sheet Music from Canada's Past
Libarary of Congress - includes sheet music archives
Essentials of Music
The Symphony - an interactive guide

Not Enough? Here's More to Explore...
Palatine Directory
BUBL Links
Online Scores for Study