Browsing along the PlanetRDF I came to Danny Ayers site which led me to the Syndication Subscription Service.

This is a good example of how localhost integration works. It shows 15 popular news-aggregation systems and provides links that enable the user to add Danny's blog to their newsreader. Interesting is, that there si no common way of doing this: There is no "add this rss feed to my newsreader" system call in the operating system or the browser.
All products implement different ways to do it. The locally instlalled systems open http ports at the locahlost and wait for http requests. The web based systems run on their servers (f.e. yahoo) and wait for the commando there.