The HELLAS2XMM 1dF Survey: a window on exotic hard X-ray selected sources

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.