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 calculationsjmol - 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 "openscience.org" 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