Among the players of Pokemon Go, there is a group of software developers who have reverse engineered part of how the game app communicates with the servers.
Using these information, they are able to develop applications to visualize actual pokemon locations around the world.
However do take note that using such programs is against the TOS of the Pokemon Go and might get your account banned.
One such site is PokeVision. They allow users to scan for nearby pokemons every 30 seconds. Pokemons that are found have a timer associated with them to indicate the time until they de-spawn.
Another application requires manual installation. https://github.com/AHAAAAAAA/PokemonGo-Map
Installing and running this map requires a bit more IT knowledge but they provide a written guide on how to install and run.
Alternatively, there's also a video guide for those who needs it.
To see more such community software developments, you can head over to https://www.reddit.com/r/pokemongodev/ where most of such devs reside.
Other than the applications listed above that directly grabs data from the Pokemon Go server, there are also players contributed maps like http://www.pokemonradargo.com/ where players submit pokemon sightings into the site so other players can make use of those information.