This review is from the perspective of someone just beginning to learn music theory, so I won't be able to credibly comment on some of the features or accuracy of this plugin.
That said, as a beginner, Tonespace has been tremendously helpful and instructive. I previously used ChordSpace to learn chords/sketch out progressions etc., but was beginning to brush up against the limitations of that plugin. I've searched for and tried out alternatives and found nothing better until Tonespace came along. This is the most robust, flexible, and intuitive scale- and chord-related MIDI plugin I've seen, period.
The concept of Tonespace is quite intuitive--chords in a scale are laid out in colored customizable grids, with a bevy of excellent preset layouts to get you started. The array of options for shaping chords is huge and I have yet to feel limited by this plugin. To be honest, there are already so many features in such a mature, useable state that it almost makes me feel guilty that Tonespace is free. The depth and quality of it is something you would expect to find as an exclusive bundled plugin in a high-caliber sequencer. Documentation is complete, clear, and instructive, with examples of how to set up the plugin in many popular sequencers, and the author has been active on the forums here helping users.
The only criticism I have of Tonespace is its GUI. It is quite sharp and user-friendly, but there are two small quirks. First, some of the grid layouts are clipped off by the edges of the plugin window--however, the author has stated this is intentional. Second, I've run into a small graphical error several times now (that could very well be the fault of something else) where my mouse pointer becomes offset while hovering over the Tonespace window. Neither of these issues detract from the stunning scope, functionality, and educational value of the plugin, however.
If you are looking for a MIDI plugin that handles scales and chords, Tonespace is in its own league beyond anything else available, and amazingly enough, it's free.
Agreed that tonespace deserves to be reviewed carefully. There's a lot of plugins out there for diverse audio effects. But not that many for MIDI effects and especially not that many no-cost ones.
From experiments with tonespace, now trying to do some similar things with my own Pure Data patches. Turns out, it's actually easier than expected, especially with help from the PdPatchRepo forum. Nice thing with that option is that it should work on a Raspberry Pi, including audio output.