I remember from my high school days, one source of "fun" was to travel around the towns and cities nearby where we lived with a laptop under our arms running programs similar to WinPCap; which searched for wireless access devices, logging the times we saw them and their strengths at those times.
It was a real shame that at the time GPRS was coming alive, but had yet to make itself known on most of these devices. Sure, you could get GPRS dongles, but none of the software we had could use those to handle our requests. Fortunately, in the modern day, this phenomena is over.
There is an app in the Google Play store that can be downloaded for free. It's called "Wifi Tracker" by Ian Hawkins (if it works why reinvent the wheel??) Which allows us to do just what we used to do in the old days only with the advantage of having GPRS ready devices to log where these devices appeared as well as when. We then feed all this information into a Google Maps API where we review it as a heat map.
The technology is really useful and can have some rather interesting technical uses, rather than just the "we can do it, so let us do so" mentality of many programmers. We can go into those later.
The front-end of the site, as well as most of the back-end, are both still in alpha testing, despite being publicaly accessible in a read-only format. They are fairly simple and straightforward at the moment. This is probably going to change as problems and errors crop in; as they have and do from time to time!
Naturally, "Wifi Tracker" isn't the only app out there, but it's the one we support at the moment. We will endeavour to support more as the need arises (I have no idea if Ian has a version of Wifi Tracker that works away from the Android platform, e.g on Windoze phones or iDevices ... but frankly I couldn't care less)