Commented keyboard links that I found to be especially useful or interesting.
Layout visualization
Keyboard Layout Editor
Create keyboard layout graphics for your reference or to share.
Strategies in Layout Design
AdNW - Aus der Neo-Welt
General considerations (in German) to find the "perfect" layout. Discusses Dvorak and other approaches and finally introduce the AdNW layout and variants. My layout is heavily influenced and based on that work and the layout optimization software they used.
The Symmetric Typing Project
An approach to using a standard keyboard more ergonomically. My layout builds on these ideas.
Neo
Ideas behind the Neo layout (in German). I favor a different approach, but the information there is still useful and interesting and also contains a good section about correct typography.
Neo, Aus der Neo-Welt, KOY: Alternative Tastaturlayouts? Warum?
Four very interesting articles (in German) on why to use alternative keyboard layouts and why and how to potentially develop an own custom layout to match personal preferences.
Layout optimization and similarity to QWERTZ
Super insightful study on how many keys need to be changed from Qwertz to improve the typing experience.
(German version available as well.)
Testing Layouts
Belegungstester
Simulating one keyboard layout with another, for German keyboards. For example, simulating on a qwertz layout how it would feel to type with Koy:
Extremely useful simulation software with detailed documentation and the possibility to create graphs of the keystroke pattern and efforts. In my opinion still one of the best if not the best options to test or develop keyboard layouts. Here an example illustrating how busy the typing on a qwerty based layout is. The connection lines show consecutive key presses.
A German version of the software and documentation is available as well.