Google Co-op caught my attention recently as a welcome addition to the social bookmarking family, although it is not really being sold as this by Google; the collaborative/collective elements are not brought to the fore. I don’t wish to suggest that Google Co-op is the “answer”, but it has a complementary approach  built more around an interest than a person.  It’s well worth a look, there is a browser add-in to send URLs to your “custom search” and a facility to download your URLs (essential if you are going to invest much time). A little disappointing is that there hasn’t been any attempt to include any categorisation or tagging within a “custom search”.