If you have a Fortran 2003 compiler, the intrinsic module iso_fortran_env defines the variables input_unit, output_unit and error_unit which point to standard in, standard out and standard error respectively.
I tend to use something like
#ifdef f2003
use, intrinsic :: iso_fortran_env, only : stdin=>input_unit, &
stdout=>output_unit, &
stderr=>error_unit
#else
#define stdin 5
#define stdout 6
#define stderr 0
#endif
in my input/output routines. Although this of course means preprocessing your source file (to do this with ifort, use the -fpp flag when compiling your source code or change the source file extension from .f to .F or from .f90 to .F90).
An alternative to this would be to write your own, non-intrinsic, iso_fortran_env module (if you don’t have a Fortran 2003 compiler), as discussed here (this link has died since this answer was posted). In this example they use a module:
module iso_fortran_env
! Nonintrinsic version for Lahey/Fujitsu Fortran for Linux.
! See Subclause 13.8.2 of the Fortran 2003 standard.
implicit NONE
public
integer, parameter :: Character_Storage_Size = 8
integer, parameter :: Error_Unit = 0
integer, parameter :: File_Storage_Size = 8
integer, parameter :: Input_Unit = 5
integer, parameter :: IOSTAT_END = -1
integer, parameter :: IOSTAT_EOR = -2
integer, parameter :: Numeric_Storage_Size = 32
integer, parameter :: Output_Unit = 6
end module iso_fortran_env
As noted in other answers, 0, 5 and 6 are usually stderr, stdin and stdout (this is true for ifort on Linux) but this is not defined by the Fortran standard. Using the iso_fortran_env module is the correct way to portably write to these units.