Java Music Theory:
http://web1.hamilton.edu/javamusic/NoteReading.html
Part of an interactive java-based series of activities designed as tutorials to help with basic music concepts. Topics of related interest here include-intervals, scales, chords and key signatures.
Circle of Fifths
http://classicalmus.hispeed.com/scales/circle.html
A handy introduction to the relation of the circle of fifths to key signatures. Site also includes easy to upload piano scale fingerings for major scales, and blank manuscript pages for practicing scales.
Tendency Tones and Minor Scales
http://www.smu.edu/~theory/minor.htm
This site contains a useful and brief introduction to the harmonic and melodic minor scales, inclusive of music examples.
The Tonal Center
http://ds.dial.pipex.com/andymilne/
The Tonal Center introduces the elements of harmony through the principle of tonality. A bit more advanced content wise than the other sites, you can find information on basic chord construction, extensions, and a number of progressions and forms. The site includes a useful and detailed treatment of modulation