Great article at speedcorp.net
John Gillett posted a really good article about Ping and Trace Route for Speedcorp.net. It has some great theory - and talks about Ping Plotter as well. It's a pretty good read - so if you're curious about how Ping, Trace Route, and Ping Plotter do their thing, you might check out that article!
Note added December, 2003: Speedcorp.net seems to be defunct now. Planetquake republished the article, which is what is linked above.