There have been a handful of PokeMon Go trackers to pop up over time, but Niantic always finds a way to disable them. For quite sometime Pokevision was the go to PokeMon tracker. After Niantic sent the developers a cease and desist letter the tool was shut down. Several other PokeMon maps and trackers stepped up to take the place of Pokevision, however many of them don’t work as well since they work strictly on user reports. You have to rely on the honesty of those posting. The fact that Niantic is also rotating Pokemon who spawn at certain locations means that Pokemon reported at one location may be different after a few hours.
If you are looking for one or two rare Pokemon to complete your Pokedex there is finally a new Pokemon tracker that actually works. The developer of FastPokeMap started reverse engineering the game after the update rolled out. The tool has only been released since August 7th and is still working for now. FastPokeMap allows you to see all the PokeMon within your range. This eliminates the time necessary to just walk around aimlessly. Use this in conjunction with GPS spoofers like FlyGPS to get the rarest of PokeMon fast!