Visual Molecular Dynamics (VMD) is a very good program for viewing all sorts of structure files.
It is free and can be obtained from http://www.ks.uiuc.edu/Research/vmd/ and installed quite easily.
You will need to register but that won’t result in enormous amounts of spam.
Once installed, it is useful to set the VMDDIR environment variable:
>export VMDDIR=/usr/local/vmd # or wherever it happens to be
and include the directory that contains the executable in your path:
The documentation can be found at the above web site or at $VMDDIR/doc/ug.pdf.
This program can be a bit slow if run across the network or on a machine that is busily calculating something.
If you are calculating on a remote machine, it can be more convenient to download the appropriate files using scp or sftp to your local machine and run vmd locally.