Booker DeWitt acquires Vigors throughout BioShock Infinite, and they grant him special abilities. You can use them to levitate enemies (Bucking Bronco) or sic a murder of crows at a foe (Murder of Crows). Etsy seller Mojave Outpost captured the Vigors into little bottles and turned them into wearable pendants. You can choose which Vigor you like the best, and it will arrive at your doorstep on an 18″ chain.
Be careful with which one you choose; they have side effects.
More photos of the shiny baubles after the break.