  We have conducted a systematic search of low-mass black holes (BHs) in active
galactic nuclei (AGNs) with broad Halpha emission lines, aiming at building a
homogeneous sample that is more complete than previous ones for fainter, less
highly accreting sources. For this purpose, we developed a set of elaborate,
automated selection procedures and applied it uniformly to the Fourth Data
Release of the Sloan Digital Sky Survey. Special attention is given to
AGN--galaxy spectral decomposition and emission-line deblending. We define a
sample of 309 type 1 AGNs with BH masses in the range $8 \times 10^4$--$2
\times 10^6$ \msun (with a median of $1.2 \times 10^6$ solar mass), using the
virial mass estimator based on the broad Halpha line. About half of our sample
of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered
here for the first time. Our new sample picks up more AGNs with low accretion
rates: the Eddington ratios of the present sample range from $<~0.01$ to ~1,
with 30% below 0.1. This suggests that a significant fraction of low-mass BHs
in the local Universe are accreting at low rates. The host galaxies of the
low-mass BHs have luminosities similar to those of $L^*$ field galaxies,
optical colors of Sbc spirals, and stellar spectral features consistent with a
continuous star formation history with a mean stellar age of less than 1 Gyr.
