ASA005 is a FORTRAN90 library which evaluates the lower tail of the noncentral student's T cumulative density function (CDF), by BE Cooper.
ASA005 is Applied Statistics Algorithm 5.
ASA005 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.
ASA076, a FORTRAN90 library which evaluates the Owen's T function, needed to compute the CDF of the noncentral T distribution.
ASA243, a FORTRAN90 library which evaluates the CDF of the noncentral T distribution.
OWENS, a FORTRAN90 library which evaluates Owen's T function.
Original FORTRAN77 version by BE Cooper; FORTRAN90 version by John Burkardt.
You can go up one level to the FORTRAN90 source codes.