I've written about a few chord plug-ins before. Xeno - Chord Explorer, a Max for Live MIDI effect, strikes me as the most musical I've encountered so far. It offers the 7 diatonic scales, has quite an extensive set of features and all that while it's free.
It does look a bit tricky at first, what with its size and all the features, but you don't really have to set anything apart from the first two dials, unless you want to. And then it offers it a lot of very nice parameters to give you interesting chords variations or even randomise them without losing the musicality. All parameters are also explained in detail on the website.
- Key - choose one of the 12 notes in the chromatic scale
- Scale - one of the 7 diatonic modes (includes Major (Ionian) as well as Minor (Aeolion))
- Quantise - correction of keys not in set scale
- Mode - behaviour if more than one key is pressed
- customise each generated chord individually with 6 parameters:
- Chord - different chord types (triad, power chord etc.)
- Inversion - different chord inversions
- Sub - root note added 1 or 2 octaves down
- Spread - note spacing (close or spread out on keyboard)
- Clone - chord copied 1 or 2 octaves up or down
- Note Filter - filter out certain notes
- Range - of notes to be passed through
- Randomise - between chosen settings