For Windows and GNOME.
OS X users will be familiar with the super Key Caps application ([link|http://guildenstern.dyndns.org/pics/random/Keyboard_Viewer.pdf|Keyboard Viewer] in 10.3):
[image|http://guildenstern.dyndns.org/pics/random/Key_Caps.jpg||It's a picture of a keyboard. The irony.||]
I can't find an application that does the same thing on either Windows or GNOME.
Ideas?