Sonic Glossary: Created for Music Hum, with encyclopedia-style entries on, and audio illustrations of, several dozen key terms. Very nicely done. May not work well on all platforms.
Grove Music Online: The online version of Grove's Dictionary of Music and Musicians, the comprehensive and authoritative dictionary of music. Type search terms in the box at upper right.
The Oxford History of Western Music: A monumental history by the great, cantankerous scholar (and Columbia graduate) Richard Taruskin, all freely available in bite-sized pieces online. A more sophisticated alternative to Kelly's text. Loads slowly.
Columbia University Libraries offer a vast collection of recorded music on CD (in 701 Dodge) and for listening online.