Other
Music Theory First Aid for Struggling Students
This site provides music theory information on such topics as rhythm, intervals, scales, chords, etc. Also, there is a fine section which describes the basic concepts of form and structure.
Open Curriculum
Open Curriculum: Pitch: Sharp, Flat, and Natural Notes
To understand the definition of a pitch, its effect on notes, and the associated symbols.
Other
Music Concepts Online: Notation: Basic Rhythm and Pitch
This site looks at rhythm, terms and concepts, which include rhythm, score, note (parts of and kinds), rests, tie. Great information.