The idea popped up when trying to filter my almost 500 del.icio.us bookmarks to find a very specific link I knew had to be there somewhere. However, only adding tags did not provide enough accuracy; I still had to skim through several pages for the right link. At that point I started looking around to see if there was any way to further limit the search, but I found nothing except for the "ignore" functionality. However, this serves a limited purpose, and it has to be turned on and off manually.
Documentation is available at the homepage.