Have a search engine that gets algorithmic search results from some search engine web services (Google search APIs). Then get social results. If del.icio.us had some web service APIs where given a set of tags, it would result a list of URLs it would be great. But instead I can either create a social URL tagging and store service (a me-too del.icio.us) and use that or screen scrape del.icio.us.
Then the real fun would be a merge algorithm to merge these two (or maybe more) lists into one ranked list.