In addition to what Doug wrote, there are gangs, (more) organized crime groups from practically any ethnicity or nation, serial killers, vampires, werewolves, vampiric werewolves, skinwalkers, wendigos, mutants, black hounds, Lemurian serpent folk, reptoids, psychotic mimes, renegade military units, corrupt cops, corrupt Feds, uncorrupt cops that misunderstand the situation, uncorrupt cops who understand that the PCs are criminal bastards just fine, Nazis, supremacists, Fascists, anarchists, supervillains, Grays, evil psionic conspirators, wizards, Deep Ones, the Sidhe, ghosts, mercenaries, evil roller derby teams, disco aficionados, hippies, smugglers, evil rock bands, dragons, robots (terrestrial, time-traveling, from outer space, from alternate dimensions, or otherwise), cyborgs, AIs, yuppies, stalkers, demons, nihilists, shapechangers, pod people, agents of SPECTRE/HYDRA/Cobra/Kobra, crazy ex-boy/girl-friends, and more.
What kind of campaign are we talking about? "Modern" is kind of broad.