abacab
Good call, Zinc!
Band in a Box is remarkable, and I already have that. But I am seeking something different than an auto accompaniment tool. With BIAB, you pick a style and plug in a chord progression, and it generates the backing tracks. I like the way BIAB will follow the chords you plug in and alter the arrangement if you change the chords.
For one thing, I just want a tool that will let me create the backing tracks, but still shift the harmonic structure of all tracks if I decide to change key or the chords, like BIAB, or Liquid Notes.
I also have Liquid Notes, which is awesome if you already have a multi-track MIDI arrangement and you want to experiment with reharmonization. It will list suggested chord variations and shift all tracks to your chosen chord. But you already need to have a piece fairly well constructed and then import that via MIDI file for it to work its magic.
"Load the MIDI tracks of your projects into Liquid Notes and have them harmonically analyzed providing you with the corresponding chord symbols and a combined piano roll view of the notes in all tracks. Regardless of the complexity and style of the arrangement, Liquid Notes provides an accurate harmonic analysis of your music. And it establishes a solid base for the changes and alterations you may want to make to selected tracks or an entire song. Lay down the foundation to a new tune in no time!
The software offers a wealth of revolutionary and creative music making features. You can transform the chords and harmonic progression to your liking, use the convenient drop-down chord selector, change the basic functionality and directionality of any chord, and spice up the flavor of your music by adding varying degrees of tension and harmonic color to your chords with the simple twist of a knob."
So what I'm saying is that I'm really looking for something to help start an idea from a blank sheet of paper, but with the structure of scales and chords to put a framework around what I plug in, in case I get stupid, LOL! 
I'm also not looking for a one key chord player, or auto-accompaniment. I have a cheap Yamaha keyboard that can do that ...
I mentioned BIAB not because of it's "auto-accompaniment" abilities but because of it's ability to generate melody and chords from nothing, literally. Wasn't sure whether you were aware of it; the Melodist. I have created a few song ideas from this and found it quick and the results good (screenshot below). Once generated I don't work within BIAB (or Realband) but export the results (usually MIDI) to your DAW and expand the ideas from there.
Also the Chord Builder is now much enhanced with possible chords suggested based on music theory...
Funnily enough I bought Liquid Notes at Christmas but didn't install it when I found out it installed LoopBe. I had previously purchased AutoTheory which as TheMaartian mentions installs a multi-channel MIDI pipe thing which is a nuisance to uninstall. I was eventually able to uninstall it through device manager (SBVmidi virtual midi driver). I also don't like how AutoTheory starts up outside of the DAW and requires adjustment of DAW MIDI settings to use it. I really want something that you just insert on a track within the DAW.
Cheers
ZT