  In a variety of models the motion of the inflaton may trigger the production
of some non-inflaton particles during inflation, for example via parametric
resonance or a phase transition. Such models have attracted interest recently
for a variety of reasons, including the possibility of slowing the motion of
the inflaton on a steep potential. In this review we show that interactions
between the produced particles and the inflaton condensate can lead to a
qualitatively new mechanism for generating cosmological fluctuations from
inflation. We illustrate this effect using a simple prototype model g^2
(\phi-\phi_0)^2\chi^2 for the interaction between the inflaton, \phi, and
iso-inflaton, \chi. Such interactions are quite natural in a variety of
inflation models from supersymmetry and string theory. Using both lattice field
theory simulations and analytical calculations, we study the quantum production
of \chi particles and their subsequent rescatterings off the condensate
\phi(t), which generates bremsstrahlung radiation of light inflaton
fluctuations \delta\phi. This mechanism leads to observable features in the
primordial power spectrum. We derive observational constraints on such features
and discuss their implications for popular models of inflation, including
brane/axion monodromy. Inflationary particle production also leads to a very
novel kind of nongaussian signature which may be observable in future missions.
We argue that this mechanism provides a simple and well-motivated option to
generate large nongaussianity, without fine-tuning the inflationary trajectory
or appealing to re-summation of an infinite series of high dimension operators.
