  Our Galaxy harbours a large population of X-ray sources of intermediate to
low X-ray luminosity (typically Lx from 10^27 to 10^34 erg/s). At energies
below 2 keV, active coronae completely dominate the X-ray landscape. However,
the nature and the properties of Galactic sources detected at energies > 2 keV
is much less constrained. Optical follow-up spectroscopic observations show
that in addition to cataclysmic variables (CVs) and very active stellar
coronae, massive stars (colliding wind binaries, quiescent high-mass X-ray
binaries and Gamma-Cas analogs) account for a sizable fraction of the Galactic
hard X-ray sources at medium flux (Fx > 10^-13 erg/s/cm^2). Cross-correlations
of the 2XMM-DR3 catalogue with 2MASS and GLIMPSE confirm the presence above 2
keV of a large population of coronally active binaries, probably of the BY Dra
and RS CVn types, in addition to many distant and absorbed massive stars. We
also report the results of a specific optical identification campaign aimed at
studying the nature of the optically faint hard X-ray sources and at
constraining the surface density of black holes (BHs), either isolated and
accreting from the interstellar medium or in quiescent binaries. Not
astonishingly, most of our sample of 14 optically faint and X-ray hard sources
are identified with CVs and Me stars. We do not find any likely counterpart in
only three cases. Our observations also allow us to put an upper limit of 0.2
BH deg^-2 at Fx = 1.3 10^-13 erg/s/cm^2 in directions toward the center of our
Galaxy. This implies a combined Bondi-Hoyle and mass accretion rate to Lx
efficiency of accretion onto black holes of less than 10^-3.
