Even if you don't actually use it as a compiler, f2c from
ftp://ftp.netlib.org/f2c/src, can be a useful tool when you're
interfacing (linking) Fortran and C.
See Generating Skeletons and Prototypes with f2c.
To use f2c for this purpose you only need retrieve and
build the src directory from the distribution, consult the
README instructions there for machine-specifics, and install the
f2c program on your path.
Something else that might be useful is cfortran.h from
ftp://zebra.desy.de/cfortran.
This is a fairly general tool which
can be used to generate interfaces for calling in both directions
between Fortran and C.
It can be used in f2c mode with
g77--consult its documentation for details.