  Parametric resonance or preheating is a plausible mechanism for bringing
about the transition between the inflationary phase and a hot, radiation
dominated universe. This epoch results in the rapid production of heavy
particles far from thermal equilibrium and could source a significant
stochastic background of gravitational radiation. Here, we present a numerical
algorithm for computing the contemporary power spectrum of gravity waves
generated in this post-inflationary phase transition for a large class of
scalar-field driven inflationary models. We explicitly calculate this spectrum
for both quartic and quadratic models of chaotic inflation, and low-scale
hybrid models. In particular, we consider hybrid models with an ``inverted''
potential. These models have a very short and intense period of resonance which
is qualitatively different from previous examples studied in this context, but
we find that they lead to a similar spectrum of gravitational radiation.
