Here: http://autotheory.net

I bought AutoTheory 3 in a sale a while back but I have never used it, I figured if i got a few good chord progressions out of it it would be worthwhile. I really must give it a decent go, it seems a reasonable program. It looks like there is an upgrade path.

$29 to upgrade from prior versions.  It's a no-go for me though.  Having to disable my controller in the DAW in order to get it routed to AT, which appears as the input ... blah.
 
Make it a VST and I'll upgrade.

I remember getting very annoyed with this program.  If I recall, it created 8 (software) MIDI interfaces and it really mucked up my music environment.  Google it and you'll see that there were lots of complaints.  Perhaps they have finally gotten it right, but I am not going near it again. 

It's not caused me any problems. I have hidden some of the virtual MIDI ports in Sonar, just to reduce the visible options. It seems to do "what it says on the tin" both driving stand alone instruments, VSTs in Sonar and recording a MIDI track in Sonar. That said I have not used it much so I may not have encountered the problems others have seen.
 
Version 3 does not seem to give me much over and above the latest capabilities in Native Instruments Komplete Kontrol. The later has been developed and now supports many more scales than it did in the beginning.
 
Toontrack's EZ-Keys has a different approach and certainly works well as a tool for experimenting with chord progressions.

Not a bad program, although a bit unintuitive at first

Don't buy... It's not VST

oockkk
Don't buy... It's not VST

That’s actually what I like about it. It’s a standalone composer tool. Creativity unleashed.

Fleer

oockkk
Don't buy... It's not VST

That’s actually what I like about it. It’s a standalone composer tool. Creativity unleashed.

Its more like having extended functionality in your controller keyboard than a plug in effect. I played around with version 3 a little more recently and while Komplete Kontrol overlaps a lot there is some interestingly different features.

Both these programs are very useful if you want to try out different scales and modes and break away from the usual major or minor scales. KK scores by lighting up the scale on your keyboard (if you have NI hardware) but both are quite educational.

oockkk
Don't buy... It's not VST

It would be more limited as a VST. Ideally it’s functionality would work best integrated into a DAW.