A technique for extracting certain parts from the HTML of a page.
There are a lot of sites that don't provide APIs, so I have a feeling it could be quite useful.
http://e8y.net/mag/013-web-scraper/
Like this site, or pulling up the price of a book on Amazon.
...It was just a Perl module!
I had been writing in PHP for a long time, so I gave up.