Music Theory
Music theory.net: Introduction to Chords
A thorough description of chords, roots, and triads. Provides illustrated examples as well as audio clips to supplement this lesson.(You may need to download Flash Player to view this site.)
Music Theory
Music theory.net: Generic Intervals
A description of intervals and how they are used in music. Provides audio clips and illustrated examples to supplement this lesson.
Other
Musical Pitch
This site offers a very technical explanation of pitch. The author's emphasis is on tuning by intervals, explaining the different spaces of intervals by name. A very complex interactive diagram is included.