Virtual keyboard

Install florence, which is a virtual (onscreen) keyboard, and lightdm-gtk-greeter:
sudo apt-get install florence lightdm-gtk-greeter
Add the following line in the [greeter]-section of /etc/lightdm/lightdm-gtk-greeter.conf:
keyboard=/usr/bin/florence --no-gnome --focus &
Change the following line:
Note the tilde characters. These are not hyphens. Also note it's "a-one-one-y", not "a-l-l-y". It took me an hour to figure that out...

That's it!

On the login screen, the keyboard can be found in the menu on the upper right corner. After login, the keyboard can be found under Applications, Utilities.