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.