FEFF is developed by the group of John J. Rehr, University of Washington, Seattle, USA.
FEFF is a real-space full multiple scattering (RSFMS) Green's function method, but it is also possible to do calculations in k-space. It is a popular code for obtaining theoretical spectra, e.g. for EXAFS and XANES.
Some useful documentation:
Will be updated with more information later on.