  An understanding of cosmic magnetism requires converting the polarization
properties of extragalactic radio sources into the rest-frame in which the
corresponding polarized emission or Faraday rotation is produced. Motivated by
this requirement, we present a catalog of multiwavelength linear polarization
and total intensity radio data for polarized sources from the NRAO VLA Sky
Survey (NVSS). We cross-match these sources with a number of complementary
measurements -- combining data from major radio polarization and total
intensity surveys such as AT20G, B3-VLA, GB6, NORTH6CM, Texas, and WENSS,
together with other polarization data published over the last 50 years. For 951
sources, we present spectral energy distributions (SEDs) in both fractional
polarization and total intensity, each containing between 3 and 56 independent
measurements from 400 MHz to 100 GHz. We physically model these SEDs, and where
available provide the redshift of the optical counterpart. For a superset of
25,649 sources we provide the total intensity spectral index, $\alpha$. Objects
with steep versus flat $\alpha$ generally have different polarization SEDs:
steep-spectrum sources exhibit depolarization, while flat-spectrum sources
maintain constant polarized fractions over large ranges in wavelength. This
suggests the run of polarized fraction with wavelength is predominantly
affected by the local source environment, rather than by unrelated foreground
magnetoionic material. In addition, a significant fraction (21%) of sources
exhibit "repolarization", which further suggests that polarized SEDs are
affected by different emitting regions within the source, rather than by a
particular depolarization law. This has implications for the physical
interpretation of future broadband polarimetric surveys.
