  If light scalar fields are present at the end of inflation, their
non-equilibrium dynamics such as parametric resonance or a phase transition can
produce non-Gaussian density perturbations. We show how these perturbations can
be calculated using non-linear lattice field theory simulations and the
separate universe approximation. In the massless preheating model, we find that
some parameter values are excluded while others lead to acceptable but
observable levels of non-Gaussianity. This shows that preheating can be an
important factor in assessing the viability of inflationary models.
