Can you point me to an example of a modern-day John Snow?

Stan Openshaw's GAM work finding a cancer cluster around an incinerator in Gateshead when everyone expected it to be on the other side of the country by the nuclear plant. You can read parts of the original paper at http://books.google.com/books?hl=en&lr=&id=qR0vfnwVuU0C&oi=fnd&pg=PA7&ots=0v2P9rhN8R&sig=tryyfcErunxpptshJ4cBagl6mDw#v=onepage&q&f=false

I nominate Ushahidi.

"The Ushahidi Platform allows anyone to gather distributed data via SMS, email or web and visualize it on a map or timeline. The goal is to create the simplest way of aggregating information from the public for use in crisis response."

I particularly like the H5N1 outbreaks time series when visualized in Google Earth. There is a KML and a blog page about it by the author, Declan Butler. To display you need a post 4.0 version of Google Earth, and it might be useful to slow down the Animation Speed in the Date and Time Options dialog.