The folks at the Simile project always seem to have a cool project or two up their sleeves. Have a look of this three-minute screencast of Sifter, a Firefox extension built by David Huynh.
It’s kind of hard to explain what it does (that’s why David made the screencast), but it’s quite fascinating. According to common wisdom, screenscraping is a bad idea. Sifter happily ignores this and takes it to its logical conclusion.
There doesn’t seem to be any download or documentation (or even website) yet, but here’s a paper about Sifter.
I think Simile should commercialise this. Not as a browser plugin, but as a web service that web site operators can use to cheaply add faceted search to their sites. I’m sure some site operators would pay for this.