  Using integrated optical spectrophotometry for 412 star-forming galaxies at
z~0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxies at z~0.1,
we investigate the H-alpha, H-beta, [O II] 3727, and [O III] 5007 nebular
emission lines and the U-band luminosity as quantitative star-formation rate
(SFR) indicators. We demonstrate that the extinction-corrected H-alpha
luminosity is a reliable SFR tracer even in highly obscured star-forming
galaxies. We find that variations in dust reddening dominate the systematic
uncertainty in SFRs derived from the observed H-beta, [O II], and U-band
luminosities, producing a factor of ~1.7, ~2.5, and ~2.1 scatter in the mean
transformations, respectively. We show that [O II] depends weakly on variations
in oxygen abundance over a wide range in metallicity, 12+log(O/H)=8.15-8.7 dex
(Z/Z_sun=0.28-1.0), and that in this metallicity interval galaxies occupy a
narrow range in ionization parameter (-3.8<log U<-2.9 dex). We show that the
scatter in [O III] 5007 as a SFR indicator is a factor of 3-4 due to its
sensitivity to metal abundance and ionization. We develop empirical SFR
calibrations for H-beta and [O II] parameterized in terms of the B-band
luminosity, which remove the systematic effects of reddening and metallicity,
and reduce the SFR scatter to +/-40% and +/-90%, respectively, although
individual galaxies may deviate substantially from the median relations.
Finally, we compare the z~0 relations between blue luminosity and reddening,
ionization, and [O II]/H-alpha ratio against measurements at z~1 and find broad
agreement. (Abridged.)
