Ukelele

Ukelele is a Unicode keyboard layout editor for the XML-format keyboard layout files in OS X (".keylayout" files). It provides a graphical interface to the layout files with support for drag-and-drop character assignments, creation of dead keys, and assignment of multi-character strings.

Ukelele is not able to reassign modifier keys (Option, Shift, Command, Control, caps lock), or change the function of special keys (Return, arrow keys, function keys, etc). Rather, it is for creating keyboard layouts for different languages or other needs.

By the way, the name of this software is intentionally misspelled (try pronouncing it "You-KEY-lay-lay"). Call it an instance of 'artistic license'.

• When a key’s output in a dead key state did not exist, and when that key only had one output (technically, no action element, just output), changing the output to the existing output caused Ukelele to assume that it was not changing anything, so nothing would happen. Worse was that you could change it to something else, and then undo would not do the correct thing. This has now been fixed.
• Some dialogs had some issues with autolayout, causing elements such as buttons to appear in the wrong place. These have been fixed, hopefully.
• Updated the manual and help to advise installing keyboard layouts for the current user only when on macOS 10.13 (High Sierra).
• Fixed a typo in the Edit menu.