Step 1: install grabc (FreeBSD: ports/x11/grabc) Step 2: install xclip (ports/x11/xclip) Step 3: assign some key in your window manager to the following sequence:

grabc 2>/dev/null|perl -pe 'chomp'|xclip -i

Voila.

Originally, I wanted to make a dockapp to click on, which would do that, but then decided that it's not worth the bother.