ASA121 is a FORTRAN90 library which evaluates the trigamma function, by BE Schneider.
ASA121 is Applied Statistics Algorithm 121.
ASA121 is available in a C version and a C++ version and a FORTRAN77 version and a FORTRAN90 version and a MATLAB version.
ASA103, a FORTRAN90 library which evaluates the digamma function.
TEST_VALUES, a FORTRAN90 library which contains sample values for a number of special functions.
Original FORTRAN77 version by BE Schneider; FORTRAN90 version by John Burkardt.
You can go up one level to the FORTRAN90 source codes.