Carnegie Mellon Wi-Fi Data: Carnegie-Mellon University has a page which shows a map on on-campus Wi-Fi activity in real time. There’s a dot for the location of every connected wireless card. This reminds me of those campuses a decade ago that had Internet-enabled soda machines that would report their temperatures so you could find the coldest can of soda on campus.
A more useful feature of this site is their reports on what wireless cards are, on average, providing the best (and worst) signals. Very cool.