Seeing a rare Pokemon in your Nearby tracker on Pokemon GO, if you have nothing better to do, can spark a long and complex manhunt that could go on for quite some time and end in aching legs. To have the Pokemon you were looking for slip away without so much as stopping by to say hello can be extremely frustrating, as can having no idea where it is. Most people by now know that a Pokemon with more paw prints in your tracker is further away, and that the indicator will flash when facing a Pokemon if you have them selected, but sometimes, those tools alone aren't enough to find that Pokemon. At times like those, a map showing where Pokemon near you are may come in handy. As it turns out, not only does such a thing exist, it's open-source.
To put a very long story in the shortest form possible, your phone sends and receives a few unique bits and bobs of data with the Pokemon GO servers, which is unique to your account. These bits of data contain things like your player stats, location, and the location of landmarks and Pokemon around you. The Pokemon data has a bit of overlap with people around you, but changes enough from user to user to be called unique. The data on Pokemon locations is in latitude and longitude form, and identifies the Pokemon by their Pokedex number. By using your account details to pull your data, or using your location to pull aggregated data, clever programmers have put together a few mapping utilities that can show where to find Pokemon.
Since the tools are open-source, they've found their way into a decent number of applications, all of which work different ways. Users should be wary of the ones that use their login information, since that can get them banned by Niantic. Likewise, some of these tools are gone now, having received cease and desist orders from on high. There are still some out there, if you look hard enough. If you head on through the source link, you'll find a quick guide to how these work in more verbose terms.