This is an excellent theoretical resource site on scales, chords and harmony. The approach the author (Andrew Milne) takes is to define Tonality. This is generally the relationship between chords, scales, notes. Typically in the domain of the major and minor scale. Milne takes this discussion to musical theory, yet comes back to some great practical approaches to finding new chord progressions (see the table under Cadential Progressions). Nice stuff here. But you do need some understanding of scale/chord relationships and basic music theory.