  We have assembled a compilation of observational Hubble parameter
measurements estimated with the differential evolution of cosmic chronometers,
in the redshift range 0<z<1.75. This sample has been used, in combination with
CMB data and with the most recent estimate of the Hubble constant H_0, to
derive new constraints on several cosmological parameters. The new Hubble
parameter data are very useful to break some of the parameter degeneracies
present in CMB-only analysis, and to constrain possible deviations from the
standard (minimal) flat \Lambda CDM model. The H(z) data are especially
valuable in constraining \Omega_k and \Omega_DE in models that allow a
variation of those parameters, yielding constraints that are competitive with
those obtained using Supernovae and/or baryon acoustic oscillations. We also
find that our H(z) data are important to constrain parameters that do no affect
directly the expansion history, by breaking or reducing degeneracies with other
parameters. We find that Nrel=3.45\pm0.33 using WMAP 7-years data in
combination with South Pole Telescope data and our H(z) determinations
(Nrel=3.71\pm0.45 using Atacama Cosmology Telescope data instead of South Pole
Telescope). We exclude Nrel>4 at 95% CL (74% CL) using the same datasets
combinations. We also put competitive limits on the sum of neutrino masses,
\Sigma m_\nu<0.24 eV at 68% confidence level. These results have been proven to
be extremely robust to many possible systematic effects, such as the initial
choice of stellar population synthesis model adopted to estimate H(z) and the
progenitor-bias.
