  We present a description of the non-equilibrium properties of a microcavity
polariton fluid, injected by a nearly-resonant continuous wave pump laser. In
the first part, we point out the interplay between the peculiar dispersion of
the Bogolubov-like polariton excitations and the onset of polariton parametric
instabilities. We show how collective excitation spectra having no counterpart
in equilibrium systems can be observed by tuning the excitation angle and
frequency. In the second part, we explain the impact of these collective
excitations on the in-plane propagation of the polariton fluid. We show that
the resonant Rayleigh scattering induced by artificial or natural defects is a
very sensitive tool to show fascinating effects such as polariton superfluidity
or polariton Cherenkov effect. We present a comprehensive set of predicted
far-field and near-field images for the resonant Rayleigh scattering emission.
