I have a strong interest in music synthesis and composition, but I don't play a musical instrument. I also love weird computer input/output devices that are optimized for specific tasks like joysticks, 3d controllers, etc. This page is a list of alternatives to standard piano-style keyboard layouts and music notation.
Most music software is built around music notation (e.g. Sybellius) or MIDI capture/block arrangement from a MIDI instrument (e.g. Reason) which also offer note-by-note editing on a keyboard piano roll. Music trackers by comparison use the approach where there is a fixed tempo and note data is streamed on a channel-by-channel format. This grew out of the 16-bit style of music making that has its roots in early 8-bit computer sound hardware like the Commodore VIC-20 and C64 computers. There's a great history of music trackers video that goes into it in great detail.
Renoise is a modern PC implementation of a music tracker, with many of the features that a traditional "digital audio workstation" (DAW) like Logic Pro, Reason, or ProTools.
Polyend Tracker is a hardware implementation of a tracker, which I haven't seen in person.
Dirtywave M8 Tracker is a hardware tracker packaged like a Gameboy! I want one! It has a synthesis engine built-in and can also emit MIDI data.
I wish that piano keyboard showed me music structure in a more visual way so I could see the chord patterns and grok them intuitively. Here's some alternatives I've found.
Live EDM built on software like Ableton Live use things like MIDI Beat Pads (e.g. Novation Launchpad) and Effects Controllers (e.g. Korg Kaoss Pad). A beat pad is a MIDI note trigger device, and an effects controller is more of a MIDI note modifier. You can use these as part of a live performance installation through Cycling 74 Max MSP as well to add note generation and video control. Software like Sonic Pi, a live music coding environment, fits with this category.
The LinnStrument is a 2D matrix MIDI keyboard that is designed to have consistent chord positioning so transposing chords always uses the same fingers. It looks like a really interesting expressive polyphonic instrument. Here's a video of Robert Linn demonstrating his invention.
This isn't a chordal or isomorphic keyboard, but the Characorder One is an interesting remapping of a traditional keyboard into a handheld ergonomic package. Intriguing!