toms571, a FORTRAN77 code which implements ACM toms algorithm 571, containing routines to determine the concentration parameter from the average modulus of a set of sample vectors, and vice versa, for both the von Mises (circular) and Fisher (spherical) distributions of directions.
The text of many ACM toms algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.
The computer code and data files described and made available on this web page are distributed under the MIT license
toms571 is available in a FORTRAN77 version.
prob, a FORTRAN90 library which contains routines to sample the von Mises and Fisher distributions.