Not necessarily a symbol. I use "setxkbmap -option caps:super && xcape -e 'Super_L=Escape'" to make tapping Caps Lock simulate tapping Escape, and holding Caps Lock and pressing another key work as Super("Windows key") + that key in X11.

(eg. Caps alone = exit insert mode in vi, Caps + Space = Super + Space for toggling the tiling layout in my window manager dwm.)