Link: A9.com > OpenSearch.
It allows users to aggregate search results by switching the search engine. On A9 site you can customize your columns by choosing your favorite search engines compatible to Opensearch. If you're a hacker (or remixer in Etech terminology), now you're free from customizing the search API query parser and you can launch or incorpolate OpenSearch platform (Aggregator) into your site. If you're a search engine developer, you will have a chance to make your search result as part of A9 search by submitting via form.
I've already submitted Bulkfeeds after making my RSS feed compatible to the format and write a damn simple definition XML. Since my feed already has an original RSS element to give items per page and page number, It took me just 30 minutes to accomplish it.
It's always interesting to see these things happen. I don't have any contract with A9 to show up my feed as part of them. It's something like "Vertical Search loosely coupled by simple XML format".
And I think A9's strategy this way is pretty neat, as individual search providers won't go this way by themselves. A9 is part of a giant player Amazon and the giant makes call for the rough standardization. As a result each player is compatible to the format which itself is "open" anyway. Pretty similar to how weblog update ping and Trackback has got common on this industry.
A question: Where can I retrieve a OPML-ish format list of OpenSearch compatible search engines submitted to A9? I don't think it's a good thing to have our search power dominated by A9 platform, as it's "Open" search.
One more: any Auto-Discovery spec to find a OpenSearch description file? It can possibly be embeded as link tags in HTML head.