  Recent Chandra and XMM-Newton surveys have confirmed that the cosmic X-ray
background is mostly due to accretion onto super-massive black holes,
integrated over cosmic time. Here we review the results obtained from the
photometric and spectroscopic follow-up observations of the 122 X-ray sources
detected by the HELLAS2XMM 1dF Survey down to a 2-10 keV flux of ~10^{-14} erg
cm^-2 s^-1. In particular, we focus on the multiwavelength properties of a few
intriguing classes of X-ray sources: high X-ray-to-optical flux ratio sources,
Type 2 quasars, and XBONGs.
