Learning Music Theory
Musictheory.net is a wonderful free site that allows you to learn, review, and test yourself on many aspects of music theory. It's full of practical information and also exercises to help you solidify your understanding. A very useful site, it is highly recommended to those of you who want to enhance your understanding of the wonderful world of music theory.
Music Theory for Practical People is a friendly, intelligently conceived, comprehensive, and fun way to learn music theory. None of the dry, dusty, crusty, approaches of those deadly college-level theory texts. Very useful topics, applicable to all instruments, are approached in a cheerful but smart and useful way, and you can go at your own pace for your own comfort in learning. You can find it at edly.com.
Clio & Euterpe, or British Harmony
clioeuterpe.org is a music rescue project launched for the purpose of converting the rare 3-volume 18th-century songbook Clio & Euterpe, or British Harmony into digital format, and sharing it with the world. Comprised of 600 pages of contemporary music, and each page decorated with beautifully engraved pictures, these files are currently being made available in high-resolution PDF format, along with midi renditions of the tunes.