Inspired by EtherPEG, Driftnet is a program which listens to network traffic and picks out images from TCP streams it observes. It is interesting to run it on a host which sees a lot of web traffic.

(Obviously, this is an invasion of privacy of a fairly blatant sort. Also, if you are possessed of Victorian sensibilities, and share an unswitched network with others who are not, you should probably not use it.)