SearchEkko is a "related posts" widget that uses visitors' search referral terms to find and present your best-matching posts.
SearchEkko looks at each visitor's HTTP referer header and identifies visitors coming from Google, Yahoo, or Bing. It then parses the referer for the search query. Finally, it uses the Yahoo BOSS index of your site to find other matching pages.
The SearchEkko widget only displays if there are other top-ranking pages on your site for the query. It will display up to four other matching results.
You can modify the CSS of the widget as described here. This is the first release of the widget, but other options will be added soon. Stay tuned!
Firstly, remember that the widget only displays if you arrive at the site from a search engine and there are in-site results for your search query (in addition to the page you arrived at). Secondly, the widget requires a space at least 480px wide in order to display -- this rules out most sidebars. (The reason for the width requirement is that it's hard to display the matching results with anything narrower.) Otherwise, it looks like you've found a bug :). Please email me about it at noah at anoaharc.com!