  (Abridged) We construct a catalog of radio sources detected by the GB6 (6
cm), FIRST and NVSS (20 cm), and WENSS (92 cm) radio surveys, and the SDSS
optical survey. The 2.7 million entries in the publicly-available master
catalog are comprised of the closest three FIRST to NVSS matches (within 30
arcsec) and vice-versa, and unmatched sources from each survey. Entries are
supplemented by data from the other radio and optical surveys, where available.
We perform data analysis a ~3000 deg^2 region of sky where the surveys overlap,
which contains 140,000 NVSS-FIRST sources, of which 64,000 are detected by
WENSS and 12,000 by GB6. About one third of each sample is detected by SDSS. An
automated classification method based on 20 cm fluxes defines three radio
morphology classes: complex, resolved, and compact. Radio color-magnitude-
morphology diagrams for these classes show structure suggestive of strong
underlying physical correlations. Complex and resolved sources tend to have a
steep spectral slope (alpha ~ -0.8) that is nearly constant from 6 to 92 cm,
while the compact class contains a significant number of flat-spectrum (alpha ~
0) sources. In the optically-detected sample, quasars dominate the
flat-spectrum compact sources while steep-spectrum and resolved objects contain
substantial numbers of both quasars and galaxies. Differential radio counts of
quasars and galaxies are similar at bright flux levels (>100 mJy at 20 cm),
while at fainter levels the quasar counts are significantly reduced below
galaxy counts. The optically-undetected sample is strongly biased toward
steep-spectrum sources. In samples of quasars and galaxies with SDSS spectra,
we find that radio properties such as spectral slope, morphology, and radio
loudness are correlated with optical color and luminosity.
