A good example is the space cadet shift. If I tap left shift, I get an open parenthesis. If I hold it and use any other key, I get a capital letter. It's a lot nicer than hitting shift+9/0 and I got used to it in about 10 minutes.
Any keyboard using the QMK keyboard firmware can do this.
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.)
Any keyboard using the QMK keyboard firmware can do this.