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The concept of trackbacks and bidirectional linking has always been really intruiging to me, but it's such a difficult problem to solve, even if you ignore the spam issue... and that's awfully hard to ignore.
Ted Nelson coined the term Hypertext over 30 years ago, and articulated its principles, most of which have been ignored in the triumph of the Web. One of the principle elements of Nelson's vision that was left out of the web is the inherent bidirectionality of hyperlinks. This just means that a link from A to B should be visible, and traversable, from B to A as well. A simple idea, but suprisingly difficult to implement if you start with the sort of document-centric model that the WWW uses.
The problem is that bidirectional linking requires a certain level of cooperation between referencers and referencees. You can't force every publisher of every document to maintain a trackback-style referencing policy or system. Fortunately, however, you can expect that someone will be in the business of crawling and indexing the web, and the topic of references, as any fan of Pagerank will tell you, falls squarely into the search domain.
LinkBack is a Firefox/Greasemonkey plugin that queries Yahoo web services as your browse the web, displaying incoming links for the web pages that you visit. It's pretty much like using the "link:" qualifier in a Google search, except that it does this automatically for you for every page that you visit, showing the results in a semi-transparent floating div over the document you are viewing.
Of course, there are privacy issues with sending all your browsing information to Yahoo, and the results are only as good as the search engine is capable, but it's a step in the right direction, and for the most part, it works. -Link.
