Hi Steve, see these previous threads for some discussion:

http://moi3d.com/forum/index.php?webtag=MOI&msg=3136.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3652.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3401.1

For something that's easy to use and inexpensive (not free though), check out SimLab composer:
http://www.simlab-soft.com/3d-products/simlab-composer-main.aspx

or Carrara:
http://www.daz3d.com/i/products/carrara

- Michael