ManualsLib is a free to use web service that includes manuals for various products.
Browse the manuals listed on the homepage or search for a product using keywords; the site has more than 465,000 manuals listed.
When a manual is found, simply click on the image thumbnail; for convenience and offline reading, download manuals as a PDF document.
A good way to find out more about products, from the manuals, before purchasing!
Via John Dalziel