Unusual keys and keyboards

The two keys PrintScrn/SysRq and Pause/Break are special in that they have two keycodes: the former has keycode 84 when Alt is pressed simultaneously, and keycode 99 otherwise; the latter has keycode 101 when Ctrl is pressed simultaneously, and keycode 119 otherwise. (Thus, it makes no sense to bind functions to Alt keycode 99 or Ctrl keycode 119.) The Pause/Break key is also special in another way: it does not generate key-up scancodes, but generates the entire 6-scancode sequence on key-down.

If you have strange keys, that do not generate any code under Linux (or generate messages like "unrecognized scancode"), and your kernel is 1.1.63 or later, then you can use setkeycodes(1) to tell the kernel about them. Once they have gotten a keycode from setkeycodes, they can be assigned a function by loadkeys.

For example, using showkey -s one sees that Microsoft keyboards use the scancode sequences (in hexadecimal) e0 5b (left Windows key), e0 5c (right Windows key), e0 5d (Menu key). Microsoft Internet keyboard also uses e0 6a (Back), e0 69 (Forward), e0 68 (Stop), e0 6c (Mail), e0 65 (Search), e0 66 (Favorites), e0 32 (Web/Home), e0 6b (My Computer), e0 21 (Calculator), e0 5f (Sleep). Use dumpkeys to see what keycodes are still unused. Typically values like 89-95 and 112-118 and 120-127 are free. Now 

        % setkeycodes e05b 125
        % setkeycodes e05c 126
        % setkeycodes e05d 127
assigns keycodes to these scancode sequences, and 
        % loadkeys
        keycode 125 = Decr_Console
        keycode 126 = Incr_Console
        keycode 127 = KeyboardSignal
        %
would make these Windows keys go to the previous or next virtual console, and let the Menu key create a fresh virtual console (in case you have something like spawn_console running).

Funkeys

Many modern keyboards have buttons or keys with labels like "Vol Up", "Eject" etc. that suggest actions rather than strings. Of course one can bind shell commands to them, but then they'll work only when you are at a shell prompt. Rick van Rein wrote a package funkey consisting of a kernel patch and a daemon. The kernel patch creates a new character device, and adds a new key type to indicate which keystrokes should be sent to this new character device. A daemon can now listen to the character device, somewhat like gpm listens to the mouse device, and perform the actions indicated in its config file. See rick.vanrein.org/linux/funkey.