If I run across a new webcomic or blog that I really like, I subscribe to its RSS / Atom feed so I can see new content. If it's sufficiently awesome, I also want to read the archives to catch up on all the old content. Subscribing is easy. Reading the archives is easy if I can do it all in one sitting; otherwise it's hard.
Google Reader would be a great way to read through a site's archives, except that it won't track read/unread status for anything older than a couple of weeks. Is there something similar that will let me browse through the entire history of an RSS feed, keep track of which posts I've read, and have a simple way to take me to the next unread post? (Also important is a way to mark a post as "unread" again, since I might not always be able to read a longer post in one sitting.)
I'm aware that the actual RSS feed only contains the last ten or twenty posts at any given time, and that's actually part of this question. To be really useful, a "read-the-archive" webapp would have to be tied into something like Google Reader that has (probably) already been watching the RSS feed since forever, and that would therefore already know all the posts that have ever come across it -- or else have some other way to scrape the site's archives or otherwise learn about its history.