So, today, I enabled the OpenSearch spec on my blog. I doubt people come here to search, but it's good technology that I think should be out there more often.
If you're using IE7, you'll notice that when you're here, your search drop-down has an orange button next to it (details on IE7's auto-discovery implentation can be found here).
Unfortunately, Firefox doesn't support OpenSearch yet (that I can find).
The way I got this working was really to read the OpenSearch spec on A9's OpenSearch How-To page. Pretty simple, actually. I created the opensearch xml file, and added the following to the header of each page on my blog:
<link rel="search" type="application/opensearchdescription+xml" title="marius dot org Search" href="http://www.marius.org/opensearch.xml" />
Simple as that.
[Update: Confirmed that Firefox does not currently support OpenSearch, but it looks like Firefox 2 will]