  We present an expanded sample of low-mass black holes (BHs) found in galactic
nuclei. Using standard virial mass techniques to estimate BH masses, we select
from the Fourth Data Release of the Sloan Digital Sky Survey all broad-line
active galaxies with masses < 2 x 10^6 M_sun. BHs in this mass regime provide
unique tests of the relationship between BHs and galaxies, since their
late-type galaxy hosts do not necessarily contain classical bulges.
Furthermore, they provide observational analogs of primordial seed BHs and are
expected, when merging, to provide strong gravitational signals for future
detectors such as LISA. From our preliminary sample of 19, we have increased
the total sample by an order of magnitude to 174, as well as an additional 55
(less secure) candidates. The sample has a median BH mass of <M_BH> = 1.3 x
10^6 M_sun, and in general the objects are radiating at high fractions of their
Eddington limits. We investigate the broad spectral properties of the sample;
55 are detected by \rosat, with soft X-ray luminosities in the range 10^40 to 7
x 10^43 ergs/sec. Much like the preliminary sample, these objects are
predominantly radio-quiet (R = f_6cm/f_4400A < 10), but 11 objects are detected
at 20 cm, with radio powers (10^21-10^23 W/Hz) that may arise from either star
formation or nuclear activity; only 1% of the sample is radio-loud. We further
confirm that, with <M_g>=-19.3 and <g-r> = 0.7 mag, the host galaxies are
low-mass, late-type systems. At least 40% show disk-like morphologies, and the
combination of host galaxy colors and higher-order Balmer absorption lines
indicate intermediate-age stellar populations in a subset of the sample.
