  We present a catalog of 857 white dwarf (WD)-M binaries from the sixth data
release (DR6) of the Sloan Digital Sky Survey (SDSS), most of which were
previously identified. For 636 of them, we complete a spectral analysis and
derive the basic parameters of their stellar constituents and their distances
from Earth. We attempt to measure fundamental parameters of these systems by
completing spectral analyses. We use a Chi^2 minimization technique to
decompose each combined spectrum and derive independent parameter estimates for
its components. Forty-one of the stellar duets in our spectroscopic sample are
optically resolved in their respective SDSS images. For these systems, we also
derive a minimum true spatial separation and a lower limit to their orbital
periods, typically which are some 10^4 yr. Spectra of 167 stellar duets show
significant hydrogen emission and in most cases no additional He i or He ii
features. We also find that 20 of the 636 WDs are fitted to be DOs, with 16
measured to have T_eff around 40,000 K. Furthermore, we identify 70 very
low-mass objects, which are secondaries of masses smaller than about 0.1 solar
masses, to be candidate substellar companions. Although various selection
effects may play a role, the fraction 6.4 % of WD-M star binaries with orbital
separations of around 500 AU is a criterion for evolutionary models of stellar
binary systems. Active M dwarfs are likely present in 155 Balmer-emitting
systems, corresponding to a fraction of 24.4 %. The excess of cool DOs is most
likely due to additional WDs in the DB-DO T_eff range, for which no detailed
fitting was completed. The trend of the M stars being closer to Earth than the
WD component is probably due to an underestimation of the theoretical M star
radii.
