This search engine indexes the entirety of the Greenkeys archives from 1998 to the present. While the Greenkeys archive has a restrictive robots.txt that disallows all respectable indexers from crawling the content there, I've obtained explicit permission to ignore these restrictions from the moderator of Greenkeys as well as the person who runs QTH.net, the supporting site behind Greenkeys and many other amateur radio-related mailing lists.
The indexer runs on a regular basis. Any new posts should be indexed and searchable within six hours maximum.
The indexer and search engine both run within the Open Search Server (OSS) suite, an open-source and full-featured engine. The original plan was to leverage Apache Nutch and Apache Solr, but OSS is a more complete (though arguably less powerful) solution with less setup, customization, and maintenance overhead.