del.icio.us
Here is the help that describes the RSS feeds that are available
» del.icio.us offers you many ways to view your own and other people's bookmarks. Understanding how del.icio.us URLs work makes it easier to see exactly the bookmarks you want.» Every user has a unique homepage:del.icio.us/geoffrey» Tags also have their own pages:del.icio.us/tag/politics» You can filter bookmarks by user and tag together:del.icio.us/geoffrey/literature» This will show all bookmarks from user geoffrey tagged as 'literature'.» Use plus signs to combine multiple tags into a tag intersection:del.icio.us/tag/politics+france+history» This will display only bookmarks that have all three of the tags 'politics', 'france', and 'history'. This also works for user/tag combinations:del.icio.us/geoffrey/literature+english
The interface is a bit spartan with almost no images at all. But the site is fully functional. I've already discovered some cool sites using this. It works in Firefox as well as IE and other browsers via a bookmarklet, pretty easy to setup. This is one cool concept.