Amazon's A9 Search engine has just added a very cool feature called OpenSearch. The idea is pretty simple, web sites can submit RSS feeds for their search results to the A9 OpenSearch web site, then users can search lots of sites from one portal, or search aggregator.

"We want OpenSearch to do for search what RSS has done for content."

"Many sites today return search results as an tightly integrated part of the website itself. Unfortunately, those search results can't be easily reused or made available elsewhere, as they are usually wrapped in HTML and don't follow any one convention. 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." -a9

I have made two of my sites OpenSearch compatible: Dealazon, and MacRead, it was pretty simple because I already supported RSS search feeds, I just had to add a few lines to my RSS feed, and create a opensearch.xml file. The instructions on the Open Search web site are pretty clear on how to do this.