  This paper describes the data release of the Sloan Digital Sky Survey-II
(SDSS-II) Supernova Survey conducted between 2005 and 2007. Light curves,
spectra, classifications, and ancillary data are presented for 10,258 variable
and transient sources discovered through repeat ugriz imaging of SDSS Stripe
82, a 300 deg2 area along the celestial equator. This data release is comprised
of all transient sources brighter than r~22.5 mag with no history of
variability prior to 2004. Dedicated spectroscopic observations were performed
on a subset of 889 transients, as well as spectra for thousands of transient
host galaxies using the SDSS-III BOSS spectrographs. Photometric
classifications are provided for the candidates with good multi-color light
curves that were not observed spectroscopically. From these observations, 4607
transients are either spectroscopically confirmed, or likely to be, supernovae,
making this the largest sample of supernova candidates ever compiled. We
present a new method for SN host-galaxy identification and derive host-galaxy
properties including stellar masses, star-formation rates, and the average
stellar population ages from our SDSS multi-band photometry. We derive SALT2
distance moduli for a total of 1443 SN Ia with spectroscopic redshifts as well
as photometric redshifts for a further 677 purely-photometric SN Ia candidates.
Using the spectroscopically confirmed subset of the three-year SDSS-II SN Ia
sample and assuming a flat Lambda-CDM cosmology, we determine Omega_M = 0.315
+/- 0.093 (statistical error only) and detect a non-zero cosmological constant
at 5.7 sigmas.
