  We present a large sample of 501 radio-selected BL Lac candidates from the
combination of the Sloan Digital Sky Survey (SDSS) Data Release 5 optical
spectroscopy and from the Faint Images of the Radio Sky at Twenty-Centimeters
(FIRST) radio survey; this is one of the largest BL Lac samples yet assembled,
and each object emerges with homogeneous data coverage. Each candidate is
detected in the radio from FIRST and confirmed in SDSS optical spectroscopy to
have: (1) no emission feature with measured rest equivalent width larger than 5
Angstroms; and (2) no measured Ca II H/K depression larger than 40%. We
subdivide our sample into 426 higher confidence candidates and 75 lower
confidence candidates. We argue that contamination from other classes of
objects that formally pass our selection criteria is small, and we identify a
few very rare radio AGN with unusual spectra that are probably related to broad
absorption line quasars. About one-fifth of our sample were known BL Lacs prior
to the SDSS. A preliminary analysis of the sample generally supports the
standard beaming paradigm. While we recover sizable numbers of low-energy and
intermediate-energy cutoff BL Lacs (LBLs and IBLs, respectively), there are
indications of a potential bias toward recovering high-energy cutoff BL Lacs
(HBLs) from SDSS spectroscopy. Such a large sample may eventually provide new
constraints on BL Lac unification models and their potentially peculiar cosmic
evolution; in particular, our sample contains a significant number of higher
redshift objects, a sub-population for which the standard paradigm has yet to
be rigorously constrained.
