Columbia University Libraries has developed the Archival Collections Portal, a unified search system helping users discover archival resources in a streamlined way. We combined the power of Lucene and Solr to search XML, parse JSON objects, create EAD-compliant documents, and deliver results in an easy-to-use interface. By reusing MARC records and employing new search engine features and techniques, we are able to bring important and hard-to-find collections to researchers and archivists. The canonical home page of the Portal is http://www.columbia.edu/library/archival/.
Take a look -- seems pretty cool!