I got to present an introduction to virtualization, for the project management group at my work. Putting it together (keynote on iPad, believe it or not) I needed some clear illustrations of computers and things. I ended up using icons from the Gnome "tango" interface. And I borrowed another one or two from KDE. If you have do technical presentations to non-technical people, they're both great sources of art to borrow.
Oh, and since I'm the official "anti establishment on principle" guy on the team, I ended the presentation with Mark Goetz' awesome powerpoint public service announcement.