The development of a new detector technology often results in major advances in observational capabilities. Recently developed microcalorimeter detector arrays will revolutionize the study of the high-energy Universe by enabling non-dispersive spectrometers with high spectral resolution (R~1000-6000) in the soft X-ray band. These new instruments will allow groundbreaking scientific observations, including studies of clusters of galaxies and probes of the accretion and outflow processes in AGNs and compact objects. I will describe microcalorimeter detector arrays that utilize superconducting transition-edge sensors (TESs), a technology that can be used not only for X-ray astrophysics but for applications across the electromagnetic spectrum (e.g., for sub-mm/infrared instruments). I will present our flight projects, including sounding rocket experiments and the Soft X-ray Spectrometer for Astro-H (launch 2014), and new observational opportunities enabled by these missions. Finally, I will discuss future instrument development prospects based on low-temperature detector arrays in the X-ray and in other wavebands.