ViennaDemag is an open source application for calculating the demagnetizing field on complex and disconnected geometries. It provides both a Truncation and a hybrid Finite Element/Boundary Element method (FEM-BEM) approach as described here. It can be used either as a standalone tool or integrated into larger simulators. Large part of the source code can also be employed as a library to be included in larger simulations.
More information can be found at https://gitlab.tuwien.ac.at/novomemlog/viennademag