OpenSearchFox is a genius idea, that you have to try to really appreciate. From simply right-clicking in a search box on any web page and choosing "Add OpenSearch plugin", it gathers all the form variables and uses them to create an OpenSearch plugin, which is installed and ready to use straight away: no need to wait for a site's authors to get round to writing an OpenSearch description file at all.
Comments
All fields are optional, email address will not be shown; no HTML, URLs are automatically hyperlinked.
definitely a step in the right direction, although I look forward to the day when these support extended search parameters (and the browsers do too)
opera 9 does this natively :)