Inspired by EtherPEG (though, not owning an Apple Macintosh, I've never actually seen it in operation), Driftnet is a program which listens to network traffic and picks out images from TCP streams it observes. Fun to run on a host which sees lots of web traffic.
Be cautious in how you use this. You not only have the chance of seeing things you may not have wanted to see, but you also have the real possibility of of invading someone's privacy. Then again, if tools such as this are included in your General Usage Policy...