Google's docs viewer now supports Zip and Rar files.
Google's docs viewer earlier supported 15 different file formats like .doc, pdf, jpg etc. Google today, added the support for ZIP and RAR files in the gmail and now you need not download the file and then view it, Just view the file and download if you need it.
Also the ZIP and RAR file embedded inside the other are also supported and can be viewed. This feature is available for mobile viewing too.