  We have investigated the evolution of a homogeneous isotropic background of
the Universe and inhomogeneous subhorizon matter density perturbations in
viable $f(R)$ models of present dark energy and cosmic acceleration
analytically and numerically. It is found that viable $f(R)$ models generically
exhibit recent crossing of the phantom boundary $w_{\rm DE}=-1$. Furthermore,
it is shown that the growth index of perturbations depends both on time and
wavenumber. This anomalous growth may explain properties of the observational
matter power spectrum from the SDSS data and can also partially counteract the
spectrum suppression by massive neutrinos making larger values of the total sum
of neutrino rest masses possible.
