  For very general scalar-field theories in which the equations of motion are
at second-order, we evaluate the three-point correlation function of primordial
scalar perturbations generated during inflation. We show that the shape of
non-Gaussianities is well approximated by the equilateral type. The equilateral
non-linear parameter f_NL^equil is derived on the quasi de Sitter background
where the slow-variation parameters are much smaller than unity. We apply our
formula for f_NL^equil to a number of single-field models of inflation--such as
k-inflation, k-inflation with Galileon terms, potential-driven Galileon
inflation, nonminimal coupling models (including field-derivative coupling
models), and Gauss-Bonnet gravity.
