ASA063 is a FORTRAN90 library which evaluates the incomplete Beta function, by KL Majumder and G Bhattacharjee.
ASA063 is Applied Statistics Algorithm 63.
ASA063 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
ASA109, a FORTRAN90 library which inverts the incomplete Beta function.
ASA226, a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
BETA_NC, a FORTRAN90 library which evaluates the CDF of the noncentral Beta distribution.
TEST_VALUES, a FORTRAN90 library which stores a few values of various mathematical functions.
TOMS179, a FORTRAN90 library which evaluates the incomplete Beta function.
TOMS708, a FORTRAN90 library which evaluates the incomplete Beta function.
Original FORTRAN77 version by KL Majumder, GP Bhattacharjee; FORTRAN90 version by John Burkardt.
You can go up one level to the FORTRAN90 source codes.