Rasmol is a nice, simple program for viewing pdb (protein data bank) files.
It is free and can be obtained from http://www.bernstein-plus-sons.com/software/rasmol/ and installed quite easily (especially if you use the binary versions).
Once installed, it is useful to set the RASMOLPATH environment variable (this directory should contain the rasmol.hlp file):
>export RASMOLPATH=/usr/local/Rasmol # 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 and the internal help is also quite useful