Humour: xscreensaver, the default (and greatest) screensaver on most free UNIX distros, may contain R-rated content, as this mail to the Fedora discussion list notes.
Much to my surprise, I stumbled across it drawing an ‘erect penis’ when I returned from lunch today. So I did some investigating:
$ strings /usr/X11R6/lib/xscreensaver/glsnake | grep penis erect penis flaccid penis