The instruments listed here and on the websites linked below follow the principle of isomorphism, the identical or similar appearance of intervals in all transpositions. This means they are key-neutral and scale-neutral. Isomorphism is also found in many alternative notation systems.
Intuitive Instruments for Improvisers (Paul Hirsh) – http://improvise.free.fr/ A website about isomorphic instruments, isomorphic panpipes, Janko keyboards, and more…
Keyboards, Accordions, Button Instruments
Generalized keyboard, a predecessor of the Janko keyboard that accommodates microtonal tunings (invented by R. H. M. Bosanquet, extended by Erv Wilson and others) – http://en.wikipedia.org/wiki/Generalized_keyboard , http://improvise.free.fr/bosanquet/bosanquet.html
Janko Keyboard Layout with two whole tone scale rows
Daskin brand Janko Keyboards (Paul Vandervoort) – http://daskin.com/
Lippens keyboard derived from Janko’s design
Chromatone brand Janko-style keyboards (Japanese company) – http://chromatone.jp/
Chromatone Lite for iPad – http://itunes.apple.com/us/app/chromatone-lite-for-ipad/id382628130?mt=8#
Dodeka Keyboard – http://www.dodeka.info/DodekaEnglish/Instruments.html
The Balanced Keyboard (Bart Willemse) – http://balanced-keyboard.com/
Chromatic Button Accordions – http://en.wikipedia.org/wiki/Chromatic_button_accordion
Musix for iPad – http://www.shiverware.com/musix.html One of the layouts is the Wicki-Hayden.
The DIY Keyboard Project (Drew Wagner) – http://diykeyboard.org Focuses on building alternative, isomorphic keyboards. It is an open-source hardware development project with the goal of making all of the CAD files, microcontroller code, etc. necessary for a hobbyist to build a digital keyboard freely available.
6-6 Colored Traditional (7-5) Keyboard (on this wiki)
Isomorphic Saxophones and Flutes (Jim Schmidt) – http://www.jsengineering.net/
Many traditional string instruments are isomorphic in their usual tunings (violin, viola, cello, double bass, mandolin, bass guitar). The guitar is mostly isomorphic in its usual tuning, with the exception being the major third between the G and B strings. Some people tune the guitar with all neighboring strings a major third apart, or all a perfect fourth apart, making the tuning completely isomorphic. Wikipedia page on regular tunings for guitar
Chromatic harp – http://www.chromatiker.de/Harfenbauer.html (Page in German)
Isomorphic Vibraphone with a Janko-style layout, and three-color pattern (Roy Pertchik) – see Tri-Chromatic Keyboard Pattern