Other
The Basics of Reading Music
This site is an overview of how to read music. The basics of notation such as note and rest values, the names of lines and spaces of both treble and bass clefs, etc. are covered. Includes downloadable audio files.
OpenStax
Open Stax: Catherine Schmidt Jones: Articulation
In music, notes often times must be articulated in certain ways. This site explains the various common articulations found in most music.