Google reader used store every entry in the feed ever, and it was my primary reason for using it. I was subscribed to tons of webcomics, and this allowed me to easily keep track of where I was. The only limitations to this was the rate they checked feeds at (I never read about any complaints about this, but also wasn't looking and wouldn't have been interested at the time), they only had history starting when the first person added a feed, and a large number of blogs and comics would only put notifications in the feed, without any actual content.