Google Reader is more than a feed reader: it's also a platform for feed caching and archiving. That means Google Reader stores all the posts from the subscribed feeds and they're available if you keep scrolling down in the interface.
A simple application for this feature is to retrieve the history of a feed for archiving purposes or to import it in a database. If you visit a blog or a news site, the feed will only contain the latest 10-20 posts, but Google Reader can show you more than that.
Just enter this URL in the address bar:
and replace FEED_URL with the address of the feed and NUMBER_OF_ITEMS with the number of historical posts from the feed.
For example, http://www.google.com/reader/atom/
feed/http://feeds.feedburner.com/GoogleOperatingSystem?r=n&n=100 should return the latest 100 posts from this blog as an ATOM/XML file.