Bookmarklets as a means of creating active intermediaries by scripting and automating the browser.
Bookmarklets have been used for RSS Autodiscovery allowing subscription to a sites RSS feed to be made automatically.
I've recently produced a FOAF Autodiscovery bookmarklet by hacking Mark Pilgrims RSS version.
John Udell's Library Lookup project uses a bookmarklet that re-writes links containing ISSN or ISBN parameters into open OPAC links, allowing a user to check whether their library contains a journal or book.
Udell also produced another form of intermediary that generated a HTML form rather than redirecting the browser. See Please Acquire. This uses, as does the availability check, a "Bookmarklet Bootloader" that loads the real code from a server -- avoiding an IE limit on bookmark size. (As well as opening up some interesting possibilities with cookies, etc).
It struck me that these kinds of browser automation/integration/scripting techniques could be classified, perhaps using a pattern language. There also seems scope for providing tools to auto-generate some of these tools given knowledge of URL structure, parameters, etc.
So I'm collating my notes here before writing up something more concrete.