  We present a detection-significance-limited catalog of 21 Sunyaev-Zel'dovich
selected galaxy clusters. These clusters, along with 1 unconfirmed candidate,
were identified in 178 deg^2 of sky surveyed in 2008 by the South Pole
Telescope to a depth of 18 uK-arcmin at 150 GHz. Optical imaging from the
Blanco Cosmology Survey (BCS) and Magellan telescopes provided photometric (and
in some cases spectroscopic) redshift estimates, with catalog redshifts ranging
from z=0.15 to z>1, with a median z = 0.74. Of the 21 confirmed galaxy
clusters, three were previously identified as Abell clusters, three were
presented as SPT discoveries in Staniszewski et al, 2009, and three were first
identified in a recent analysis of BCS data by Menanteau et al, 2010; the
remaining 12 clusters are presented for the first time in this work. Simulated
observations of the SPT fields predict the sample to be nearly 100% complete
above a mass threshold of M_200 ~ 5x10^14 M_sun/h at z = 0.6. This completeness
threshold pushes to lower mass with increasing redshift, dropping to ~4x10^14
M_sun/h at z=1. The size and redshift distribution of this catalog are in good
agreement with expectations based on our current understanding of galaxy
clusters and cosmology. In combination with other cosmological probes, we use
the cluster catalog to improve estimates of cosmological parameters. Assuming a
standard spatially flat wCDM cosmological model, the addition of our catalog to
the WMAP 7-year analysis yields sigma_8 = 0.81 +- 0.09 and w = -1.07 +- 0.29, a
~50% improvement in precision on both parameters over WMAP7 alone.
