A9's new OpenSearch program out today allows anyone with search results to create their own "button" that can be added to the A9 search engine, allowing users there to get custom specialty or vertical search results.
A number of new buttons A9 users can add have already been posted on the site, in conjunction with the announcement that was made at the Etech conference today from Amazon chief Jeff Bezos. You can add things like a NASA search, a Flickr Photo Search or even a search for chess games. It's pretty slick.
Just because you create a button doesn't mean that users will automatically get them, of course. They'll have to use the page above to find options, then choose to add them. And that may happen only if you've created the search and submitted it. How to create and submit is covered in the help documents for OpenSearch.
From what I can tell, OpenSearch is built around adding some minimal extensions to RSS, which seems to be Amazon's way of escaping criticism that it's trying to do something proprietary and gain acceptance. Indeed, while A9 will tap into these search RSS feeds, potentially anyone else could, as well, as it explains:
OpenSearch offers an alternative: an open format that will enable those search results to be displayed anywhere, anytime. Rather than introduce yet another proprietary or closed protocol, OpenSearch is a straightforward and backward-compatible extension of RSS 2.0, the widely adopted XML-based format for content syndication.
For a bit more, see John Battelle's post that covers the announcement and has some comments for A9 head Udi Manber: A9 Launches "Open Search" - Vertical Search, Syndicated.