Delicious (del.icio.us) is a social bookmarks manager.
Okay, big deal. Bookmark management isn't exactly rocket science. Anyone who's been on the web for any length of time remembers all sorts of desktop and web based tools to manage your bookmarks when the Web first arose.
Simple. The context has significantly changed since let's say, 2000 even. First, the concept of Web services, especially REST style services, now has traction. So del.icio.us has a Web native programming ecology to embed itself. Second, experimentation with large scale social Web based tools (acknowledging all the research on Usenet) is still in its infancy. Meta Weblog Services is not quite applicable here, but something meta on top of this infrastructure could easily be built.