  We present the "C4 Cluster Catalog", a new sample of 748 clusters of galaxies
identified in the spectroscopic sample of the Second Data Release (DR2) of the
Sloan Digital Sky Survey (SDSS). The C4 cluster--finding algorithm identifies
clusters as overdensities in a seven-dimensional position and color space, thus
minimizing projection effects which plagued previous optical clusters
selection. The present C4 catalog covers ~2600 square degrees of sky with
groups containing 10 members to massive clusters having over 200 cluster
members with redshifts. We provide cluster properties like sky location, mean
redshift, galaxy membership, summed r--band optical luminosity (L_r), velocity
dispersion, and measures of substructure. We use new mock galaxy catalogs to
investigate the sensitivity to the various algorithm parameters, as well as to
quantify purity and completeness. These mock catalogs indicate that the C4
catalog is ~90% complete and 95% pure above M_200 = 1x10^14 solar masses and
within 0.03 <=z <= 0.12. The C4 algorithm finds 98% of X-ray identified
clusters and 90% of Abell clusters within 0.03 <= z <= 0.12. We show that the
L_r of a cluster is a more robust estimator of the halo mass (M_200) than the
line-of-sight velocity dispersion or the richness of the cluster. L_r. The
final SDSS data will provide ~2500 C4 clusters and will represent one of the
largest and most homogeneous samples of local clusters.
