"xkeycaps" is a graphical front-end to "xmodmap". It opens a window
that looks like a keyboard. Moving the mouse over a key shows what
KeySyms and Modifier bits that key generates. Clicking on a key
simulates KeyPress/KeyRelease events on the window of your choice. It
is possible to change the KeySyms and Modifiers generated by a key
through a mouse-based interface. This program can also write an input
file for xmodmap to recreate your changes in future sessions.
Jamie Zawinski <email@example.com>