Massive black holes (MBHs) are inextricably connected to the formation of massive galaxies, but their formation, evolution, and specific effects on their hosts are not clearly understood. Cosmological simulations of galaxy formation, including prescriptions for MBH formation, mergers, accretion, and feedback, are a unique way to shed light on this issue. I adopt a novel approach to forming seed black holes in galaxy halos which is dictated directly by the physics of primordial, zero-metallicity gas and motivated by physical models of massive black hole formation. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems. I also predict a population of ``wandering'' MBHs in the halos of massive galaxies, which are the remnant cores of tidally stripped satellite galaxies. These objects may be observed as off-nuclear ultraluminous X-ray sources if the cores retain a gas reservoir and are perturbed in some way, or if a nearby star is tidally disrupted.