  Recent hard X-ray surveys have proven to be effective in discovering large
numbers of X-ray sources that, despite the likely association with active
nuclei, appear to be characterized by "peculiar" properties. Among these
"exotic" source populations, we will focus on the nature of two classes of hard
X-ray sources: those characterized by high X-ray-to-optical flux ratios -- a
fraction of these are associated with the rather elusive Type 2 quasars -- and
the X-ray bright optically normal galaxies, also known as XBONGs.
