The ShowQuoteInContext Safari extension implements a nifty extension to standard HTML links: the ability to create citation links that jump directly to quoted content and highlights it.
Instead of taking the user to the top of a book-length web page and leaving them to maybe search out the content you cited, a ShowQuote link takes them directly to the quoted text. This adds a LOT of useful functionality, similar to citations in books and magazines with page numbers that take you directly to the relevant part of the source material.
Currently linking to specific content in a web page requires an anchor to link to. ShowQuote links don't require anchors, so you can link to any text you want to.
The programmer who developed the plugin, Jean Tantra, also points out that ShowQuote links are useful for machine learning, as they give programs an easy way to show you where they learned something.
ShowQuote links work on most web pages (not redirects or iframes). Browsers not running the plugin ignore the special sauce and treat ShowQuote links like any other link.
I would love to see ShowQuote links included in the new HTML standard. Which would be easy to do since the work is basically done.
More info: http://algorithmicartisan.com/showquote/