I think Jon Udell's LibraryLookup Bookmarklets represent a good example of “Smart Tag”-like technology that is done in a cooperative and non-intrusive way.
Jon Udell's LibraryLookup also puts the lie to Charles Cooper's news.com piece claiming that IE dominance has thwarted innovation. Jon proved that one guy can extend the browser to do something very useful to people's lives, “semantic”, and futuristic — all without ever having to fire up a compiler. The only things getting in the way of browser innovation right now are lack of imagination, lack of volition, and the giant sucking sound of people wasting their lives trying to rewrite software that's already matured (like word processors and file servers). [Better Living Through Software]