  The small-scale power spectrum of the cosmological matter distribution
together with other cosmological data provides a sensitive measure of the hot
dark matter fraction, leading to restrictive neutrino mass limits. We extend
this argument to generic cases of low-mass thermal relics. We vary the cosmic
epoch of thermal decoupling, the radiation content of the universe, and the new
particle's spin degrees of freedom. Our treatment covers various scenarios of
active plus sterile neutrinos or axion-like particles. For three degenerate
massive neutrinos, we reproduce the well-known limit of m_nu < 0.34 eV. In a
3+1 scenario of 3 massless and 1 fully thermalized sterile neutrino we find
m_nu < 1.0 eV. Thermally produced QCD axions must obey m_a < 3.0 eV,
superseding limits from a direct telescope search, but leaving room for solar
eV-mass axions to be discovered by the CAST experiment.
