Developed by Reddit user icer5k, the app's as straightforward as it gets: all you have to do is download either the OSX or Windows-compatible version from his GitHub page, unzip the file, and run the program. You'll be prompted to sign into your Pokémon Go account (either with Google or your Pokemon Trainer), and once that's done, you're taken to a map that zeroes in on your location and populates with all the Pokémon in your vicinity. Not only that, you can click on a Pokémon to see how long it'll remain at its current location. It's astonishingly accurate.
Unfortunately, the app's only available on your desktop computer for now, so you'll have to do your research at home before heading out into the world -- or, if you're a real badass, take your laptop and use a mobile WiFi hotspot for total Poké-domination. Does this remove some of the mystery from the game? Yes. Will you use it anyway? Also yes.