A few years after the first observation of Bose-Einstein condensation  in quantum gases of exciton-polaritons in semiconductor microcavities,  these systems turn out to be very promising candidates for the study of  many-body effects in a novel non-equilibrium context.  In this talk, I shall first present the recent experiments that have  recently demonstrated superfluid hydrodynamics effects in polariton  gases and I shall try to address a few conceptual questions on the  meaning of superfluidity and Bose-Einstein condensation for non-equilibrium systems.  In the second part, I will review a few most significant theoretical  advances towards the generation of strongly correlated states of photons, from Tonks-Girardeau gases of impenetrable photons in 1D, to  quantum Hall liquids in the presence of artificial magnetic fields.