I find really useful to switch keyboard layout while holding a key. That way I can easily type an English while writing in Greek. Left Windows key is good option for me as I don’t really need it on my Arch Linux powered laptop.

Can be easily setup in /etc/X11/xorg.conf.d/01-keyboard-layout.conf:

Section "InputClass"
        Identifier "keyboard-layout"
        Driver "evdev"
        MatchIsKeyboard "yes"
        Option "XkbLayout"  "us,gr"
        Option "XkbOptions" "grp:alt_caps_toggle,grp:lwin_switch"
EndSection

A full list is available in /usr/share/X11/xkb/rules/xorg.lst