Ways to view DjVu files
Various free software for viewing DjVu files is available (note some platforms have more than one alternative). The web-browser plugin and standalone viewers are stable and blindingly fast.
I usually prefer PDF for printing and DjVu for online viewing. I recommend DjVu all the way for scanned documents, but none of my publications are.
Ways to view PDFs
Wikipedia maintains a list of PDF viewers.
Google’s document viewer is a plugin-free way to quickly check a pdf document on the web. I have added GoogleViewer links for some of my publications. If you want, you can add a GoogleViewer icon to the end of PDFs on all websites.
Finally not a viewer, but amazingly useful, pdftk lets you split, merge and maintain PDF files in various ways. A fork that's easier to get working with current linux distributions is pdftk-java.