  We determine the optical luminosity function of virialized systems over the
full range of density enhancements, from single galaxies to clusters of
galaxies. The analysis is based on galaxy systems identified from the Nearby
Optical Galaxy (NOG) sample, which is the largest, all-sky catalog of
objectively- identified bound objects presently available. We find that the
B-band luminosity function of systems is insensitive to the choice of the
group-finding algorithms and is well described, over the absolute-magnitude
range -24.5 < M < -18.5, by a Schechter function with alpha=-1.4, Mstar=-23.1
and phistar=4.8 10^{-4} Mpc^-3 The characteristic luminosity of virialized
systems is ~3 times brighter than that of the luminosity function of galaxies.
Our results show that half of the luminosity of the universe is generated in
systems with L<2.9 Lstar and that 10% of the overall luminosity density is
supplied by systems with L>30 Lstar. We find a significant environmental
dependence in the luminosity function of systems, in the sense that overdense
regions, as measured on scales of 5 Mpc, preferentially host brighter, and
presumably more massive, virialized systems.
