  A fast Poisson solver software package PoisFFT is presented. It is available
as a free software licensed under the GNU GPL license version 3. The package
uses the fast Fourier transform to directly solve the Poisson equation on a
uniform orthogonal grid. It can solve the pseudo-spectral approximation and the
second order finite difference approximation of the continuous solution. The
paper reviews the mathematical methods for the fast Poisson solver and
discusses the software implementation and parallelization. The use of PoisFFT
in an incompressible flow solver is also demonstrated.
