Can I use (only) the Super key as a shortcut?

Yes, you can. You can fill with Super_L to the shortcut you want, for example to use win key only for launching gnome menu :

open terminal, type :

gconftool-2 –set /apps/metacity/global_keybindings/panel_main_menu –type string “Super_L”

The gnome menu will lauch with windows key only.

The Super key is a so-called modifier key, i.e., it is only meaningful in combination with some other key (it "modifies" the other key to report Super+key instead of just key). This makes it impossible to bind it to a shortcut with the System->Preferences->Keyboard shortcuts GUI without a tweak.

You can tell the system to not consider Super a modifier key any longer; then you will be able to map it to any shortcut you want.

1. In a terminal, type the command xmodmap; it will output a few lines -- look for the one that has Super_L and Super_R in it. It should look like this:

   mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce)
   

   See the mod4 at the beginning of the line? That means that Super_L and Super_R (left- and right- Super keys) are bound to the 4th modifier (there are 8 in total).

2. If you want to be able to bind the Super_R key, give the command (note the quotes!):

   xmodmap -e 'remove mod4 = Super_R'
   

   This tells X11 that right-Super should no longer be a modifier.

Note: These settings will not survive a reboot; you can make them permanent by creating a text file .Xmodmap in your home directory, and writing a single line to it:

 remove mod4 = Super_R