What software do you recommend for creating or editing a keyboard layout?
I use Windows, but feel free to suggest for Linux and other OSes too.
For Windows, Keytweak and Microsoft Keyboard Layout Creator do what I need (Keytweak is fast and easy with a nice UI, MSKLC is more involved and requires compiling keyboard layouts, and is occasionally flaky)...
For OS X, I'd highly recommend Ukelele:
For full-power keyboard assignment in OS X, Quicksilver triggers and QS in general.
My 2 Swiss-Army knife tools for keyboard modifications: