The bulk of recent cosmological research has focused on the adiabatic cold
dark matter model and its simple extensions. Here we present an accurate fi
tting formula that describes the matter transfer functions of all common va
riants, including mixed dark matter models. The result is a function of wav
enumber, time, and six cosmological parameters: the massive neutrino densit
y, number of neutrino species degenerate in mass, baryon density, Hubble co
nstant, cosmological constant, and spatial curvature. We show how observati
onal constraints-e.g., the shape of the power spectrum, the abundance of cl
usters and damped Ly alpha systems, and the properties of the Ly alpha fore
st-can be extended to a wide range of cosmologies, which includes variation
s in the neutrino and baryon fractions in both high-density and low-density
universes.