Solr is an open source enterprise search server based on the Lucene Java search library, with XML/HTTP and JSON AP
Mozdex -- open source search engine based on Nutch
http://www.mozdex.com/
http://www.nutch.org/
interesting readings:
http://www.linux.org/news/2004/04/09/0002.html
http://www.technewsworld.com/story/31653.html
Java port of the popular PHPNuke