Spinor Bose Einstein condensates are employed as nearly quantum-limited sensors. In addition, this magnetic quantum fluid serves as a platform for studies of quantum dynamics. Using a spin sensitive imaging method, its vector magnetization is measured in situ with high spatial and temporal resolution. As a first application toward metrology, ⁸⁷Rb spinor Bose Einstein condensates are employed as precision magnetic microscopes. The demonstrated field sensitivity of 8.3 pT/Hz^1/2 over a measurement area of 120 μm² marks an improvement over the low-frequency field sensitivity of modern SQUID magnetometers. Second, dynamical instabilities in a ⁸⁷Rb F = 1 spinor Bose Einstein condensate are used as a parametric amplifier of quantum spin fluctuations. The performance of this spin amplifier is observed to be nearly quantum-limited at a gain as high as 30 dB. Third, the possibility of preparing spin squeezed states is investigated theoretically, projecting 10 dB and 17 dB of spin squeezing to be attainable in the multi-mode and single-mode regimes. In addition to serving as quantum-limited sensors of spin, spinor condensates provide a compelling opportunity to access the dynamical properties of a magnetic superfluid. For example, the evolution of helical spin textures in F = 1 ⁸⁷Rb Bose condensates is observed to be significantly affected by dipole-dipole interactions, presenting a new experimental arena for studies of dipolar quantum fluids.