Justice City is a setting for a superheroic world with recent emergent superheroes. It provides a useful, if general, backdrop for a campaign or just for a place to visit.
Justice City Superpowered: A Superhero Setting Book and Campaign Installment is a 32-page PDF (27-pages if you remove the cover, OGL pages and ad) for the Mutants & Masterminds (3rd Edition) RPG written by Ethan Parker and published by Gamer’s Haven Publishing. This is part of the Gamer’s Haven Superpowered Superhero Roleplaying line.