  The computation of interfacial free energies between coexisting phases
(e.g.~saturated vapor and liquid) by computer simulation methods is still a
challenging problem due to the difficulty of an atomistic identification of an
interface, and due to interfacial fluctuations on all length scales. The
approach to estimate the interfacial tension from the free energy excess of a
system with interfaces relative to corresponding single-phase systems does not
suffer from the first problem but still suffers from the latter. Considering
$d$-dimensional systems with interfacial area $L^{d-1}$ and linear dimension
$L_z$ in the direction perpendicular to the interface, it is argued that the
interfacial fluctuations cause logarithmic finite-size effects of order $\ln
(L) / L^{d-1}$ and order $\ln (L_z)/L ^{d-1}$, in addition to regular
corrections (with leading order $\text{const}/L^{d-1}$). A phenomenological
theory predicts that the prefactors of the logarithmic terms are universal (but
depend on the applied boundary conditions and the considered statistical
ensemble). The physical origin of these corrections are the translational
entropy of the interface as a whole, "domain breathing" (coupling of
interfacial fluctuations to the bulk order parameter fluctuations of the
coexisting domains), and capillary waves. Using a new variant of the ensemble
switch method, interfacial tensions are found from Monte Carlo simulations of
$d=2$ and $d=3$ Ising models and a Lennard Jones fluid. The simulation results
are fully consistent with the theoretical predictions.
