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

weblab lite - displays structures, handles crystals (periodic boundary conditions)

jmol -     displays structures; can read Gaussian98 .log files and animate vibrations. Note this
                programm is on the "openscience.org" web page, where there are lots of
                other useful applications.

dynamo -- a set of programs for doing MC and MD simulations as well as semi-empirical
                electronic structure calculations

gamess - a very flexible electronic structure package

Tinker - a flexible program for carrying our MC and MD simulations. Allows for polarizable
                force fields