Egothor
Egothor is an Open Source, high-performance, full-featured text search engine written entirely in Java. It is technology suitable for nearly any application that requires full-text search, especially cross-platform. It can be configured as a standalone engine, metasearcher, peer-to-peer HUB, and, moreover, it can be used as a library for an application that needs full-text search.

Jakarta Lucene
Jakarta Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search, especially cross-platform.

Nutch
Nutch is a powerful web-search engine that is based on the Lucene index engine. It adds such features as a crawler, link-graph database, parsers for HTML and other document formats, etc.

Oxyus
Oxyus is an open source search engine written in 100% Java. Oxyus is aimed to provide that search button to your website in an easy way.

Zilverline
Zilverline is what you could call a 'Reverse Search Engine'. It is a search engine that offers web access to your personal or intranet content.