The NCAR Command Language (NCL) is a scripting language that is useful for manipulating files and creating visualizations. It also has functions that have been added to support the NCAR models (e.g. CAM, CCSM, ...). NCL can be donwloaded for free form the Earth System Grid, and it is supported on several platforms including MacOSX, Linux, Solaris, and Windows running Cygwin/X.
The NCL home page has documentation, an excellent gallery showing how to make plots and many other examples. NCL takes a little getting used to and has its limitations, but it is very powerful and I find it to be the easiest way to work with NETCDF files. You can put a resource file in your home directory that changes NCL's default behavior. I recommend installing the hluresfile defined by NCAR into the file ~/.hluresfile.
There are several scripts that have been checked into the 'tests' directories for some of the NCAR modeling branches in our subversion repository. The file names end in a '.ncl' suffix. My understanding of NCL and the language itself has evolved or time, so these examples may not be the best way to use the language, but they do show how to do some useful tasks like modifying an initial conditions file. creating an off-line meteorology file, calculating the mass of a tracer, and doing a variety of simple plots.
NCL 5.1.0 binaries for Mac OS X have dependencies for Gnu Fortran (gfortran) libraries in /usr/local/gfortran. Gnu Fortran can be downloaded here.