  Peculiar velocities are an important probe of the growth rate of mass density
fluctuations in the Universe. Most previous studies have focussed exclusively
on measuring peculiar velocities at intermediate ($0.2 < z < 1$) redshifts
using statistical redshift-space distortions. Here we emphasize the power of
peculiar velocities obtained directly from distance measurements at low
redshift ($z \lesssim 0.05$), and show that these data break the usual
degeneracies in the \Omega_{m,0} -- $\sigma_{8,0}$ parameter space. Using only
peculiar velocity data, we find $\Omega_{m,0} = 0.259\pm0.045$ and
$\sigma_{8,0} = 0.748\pm0.035$. Fixing the amplitude of fluctuations at very
high redshift using observations of the Cosmic Microwave Background (CMB), the
same data can be used to constrain the growth index $\gamma$, with the
strongest constraints coming from peculiar velocity measurements in the nearby
Universe. We find $\gamma = 0.619\pm 0.054$, consistent with LCDM. Current
peculiar velocity data already strongly constrain modified gravity models, and
will be a powerful test as data accumulate.
