Here is a list of public domain (or shareware) software that you might find to be of use.

arguslab - includes a builder, nice graphics, force fields, semiemprical and HF electronic
                structure calculations

jmol -     displays structures; can read output files from several electronic structure programs and
              can display structures, vibrations, orbitals, etc. .log files and can animate vibrations. Note
              that this program is at "" web page, where there are many other useful applications.

openbabel – this is a very useful program for converting between different formats for geometries,
               including those for periodic systems (version 1.6 is the last version that supported Gaussian z-matrices)

gamess - a very flexible electronic structure package

webmo – builder plus viewer, that interfaces with several electronic structure codes

molden – very useful for viewing orbitals, charge densities, etc. (ca build structures using z-matrices)

molworks – nolecular builder and viewer

jmoleditor – molecular building and other capabilities

pymol – a very useful viewer, animator