Massachusetts Institute of Technology
Mit: Open Course Ware: Schubert to Debussy
A complete undergraduate-level online course on Western music history for the period 1815-1915. Includes course assignments.
Other
Richard Wagner and His Operas
A comprehensive site detailing the operas of Richard Wagner. This site includes a biography, audio, video and many other details about Wagner and his operas.
Stanford University
Stanford Encyclopedia of Philosophy: Friedrich Nietzsche
A thorough, insightful overview of Nietzsche's life and works. A superb introduction.
Other
The Aria Database
The searchable Aria Database features over 1,200 arias from 170 operas by 60 composers. It includes lyrics, translations, synopses of operas, and MIDI files.