Catch 'Em Go is a crowdsourced map of important Pokémon data--where you are, where Gyms and PokeStops are around you, where other trainers are, and nearby Pokemon, as well as the cool community meetups and events happening in your area.
I'd like to note that because the information is crowdsourced, and not a series of server-loading calls, it's unlikely to get shut down. Yay for playing by the rules!
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I love how this is put in the 'Get Shit Done' section :)
WeKiki