searchplugins.net is a repository for openSearch plugins for IE7 and Firefox2. There is a generator to create the plugin and the result will be stored on the site. This is an option for bloggers who want to provide an autodiscovery link in their template but do not have a method for uploading an xml file to their blog site.
Check the list of available plugins first. Plugins can be tagged.
The FAQ page covers some good basic information about openSearch plugins.
Inn0vate's search plugin is there, if you want to visit the site. Although I'm probably the only one who'll ever bother searching this blog.
Alternatively, an autodiscovery link is embedded in the template for this blog, so you should be able to add this search plugin directly from the search tool in either IE7 or Firefox2. Look for an orange glow around the magnifying glass in IE7, or a greenish glow around the icon in Firefox2.