New Ajax social bookmark URL(you can see with what tags one have bookmarked you) and tag search. Says it will be adding more social bookmarking query services almost daily.
"In folksonomies (...) we get to discover content based on who is tagging it. This is powerful because now we can judge content in terms of who (...), and not just how relevant it might be to some algorithm that doesn’t take into account who-knows-who."