From the "I can't believe it's Java!" department comes Visual Route. If you like mapping and/or if you like networking you'll probably get a kick out of this app. Nicely done interface with all kinds of interesting goodies. I tracked a spam email back through VA, GA, MD, London, Paris, Frankfurt, and finally to Infostrada hosting in Italy.
I can't imaging being online much without this valuable and fun tool! I've used it for years. They now have another product specifically aimed at getting information about spam sent to you.