Summed up in one sentence, driftnet is a software for linux, designed to monitor and display JPEGs and GIFs as well as MPEG audio streams floating through your network. You need libpcap, libjpeg, libungif, and GTK to build it. It's similar to EtherPEG, which does more or less the same for Macintoshs. If you have the need to convince others in your network to use https, showing them driftnet working should do the trick..
Where to get it
Take a look at the official page or use CVS:
cvs -d :pserver:firstname.lastname@example.org:/home/chris/vcvs/repos login cvs -d :pserver:email@example.com:/home/chris/vcvs/repos co driftnet
As it says on the official page, the password for CVS is anonymous.