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.
Sea of Stars RPG Design Journal 