"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.
Any site that has content - and a search box - can choose to return results in OpenSearch RSS. This includes travel sites, classifieds, encyclopedias... If you can provide search results for something, it probably can fit into the OpenSearch model. Returning OpenSearch results is easy - the format is the standard set of XML elements, plus three additional elements designed to support navigation between pages."
Update: The new OpenSearch homepage.