  I present a cluster Monte Carlo algorithm that gives direct access to the
interface free energy of Ising models. The basic idea is to simulate an
ensemble that consists of both configurations with periodic and with
antiperiodic boundary conditions. A cluster algorithm is provided that
efficently updates this joint ensemble. The interface tension is obtained from
the ratio of configurations with periodic and antiperiodic boundary conditions,
respectively. The method is tested for the 3-dimensional Ising model.
