hi gollu, you can see opds catalogs in a lot of sites, and you can even make your own ; there are a few sites listed in the article above, such as feedbooks (this is how feedbooks can be integrated to the stanza or aldiko reading application, or the GoodReads site, for example). if you go to goodreads, on the page of any pd or cc book which is available on feedbooks, for example here
http://www.goodreads.com/book/show/6...-and-prejudice you'll see a button "download" ; that is the feedbooks opds catalogue integrated into the site.
another use is demonstrated by
calibre2opds, which allows you to create an opds catalogue of your own books in calibre, import that to stanza on your iphone, and have easy access to any of your books at any moment.
you might want to read the
feedbooks blog post i linked for more explanation.