OpenSearch RSS > OpenSearch
OpenSearch is a collection of technologies, all built on top of popular open standards, to allow content providers to publish their search results in a format suitable for syndication.

Dave Winer pointed to this an it seems intriguing. If I read it right OpenSearch RSS is an extension of RSS 2.0 that describes formating for search results such that the results returned are in RSS instead of whatever the engine would normally return.

This sounds interesting, but most search results can already be easily expressed as RSS without the extension.