Another feature of the Google Docs Viewer Website mentioned above is that you can create your own URLs that point to files online, and send them to others so they can instantly view the file by clicking the link.
Just make the first part of the URL read as follows:
http://docs.google.com/viewer?url=
And then add the URL of the file straight afterwards, including the http:// component. If I wanted somebody to be able to view the Microsoft Word file located at http://keirthomas.com/dump/testfile.docx, for example, I'd send them the following URL:
http://docs.google.com/viewer?url=http://keirthomas.com/dump/testfile.docx