OpenSearch is a collection of simple XML formats for sharing search results, that was originally developed by A9, a company founded by Amazon.com. A9 acts as a search mediator: You pick your favorite search engines, and A9 sends your queries to these engines, aggregates the results, and done, you have your own personal view of the web!
Many search engines provide some kind of OpenSearch or RSS-like search these days, for instance, here's an ego search on Yahoo. But, OpenSearch is just as useful on a much smaller scale, for instance for searching these pages for information on SIKS (the Dutch School for Information and Knowledge Systems).
Interestingly, as an afterthought on this post. OpenSearch seems to be taken up by many important sites, for instance Twitter: http://search.twitter.com/search.atom?q=twente, or Flickr: http://api.flickr.com/services/feeds/photos_public.gne?tags=universiteit,twente Although, they do not provide the OpenSearch description file (see: http://www.cs.utwente.nl?feed=opensearch-description), nor do they follow the standard exactly, for instance: http://en.wikipedia.org/w/api.php?format=xml&action=opensearch&search=twente