hhkb_karabiner
Provides some Karabiner key remappings for the Happy Hacking Keyboard Professional 2 (HHKB) that I've found useful.
Command Escape to Command `
This is primarily for application switching. Using my left hand for Command-Tab to move forward through the list left me always wanting to be able to use Command-Esc to move backward through the list (since ESC is where the backtick is on other keyboards).
Disable ESC for Exit Fullscreen in Safari and Apple Mail
I find it super annoying that ESC exits fullscreen mode in Safari and Apple Mail yet Apple provides no way to disable that behavior.
For Safari, this remaps ESC to Option-ESC, which means ESC will still be sent to pages in the browser (for dismissing popups, etc).
For Apple Mail this disables ESC entirely. Which hasn't seemed to cause any ill effect.
Acknowledgements
I'm sure I used The Google to find examples of similar remappings but I don't remember what I found or how closely what I ended up with resembles whatever it was I came across. If you came up with any of these remappings first and want credit, please let me know.