Euphonic
Euphonic is a Python package that can efficiently calculate phonon bandstructures and inelastic neutron scattering intensities from a force constants matrix (e.g. from a .castep_bin file). Euphonic can also do simple plotting, and can plot dispersion and density of states from precalculated phonon frequencies (e.g. CASTEP .phonon files).
The main way to interact with Euphonic is via the Python API, but there are also some useful command line tools. See the links below for more information.