A cross-platform, full-featured text search engine written entirely in Java. It can be configured as a standalone engine, metasearcher, peer-to-peer HUB or used as a class library.
A C++, GPL-licensed search engine developed at the University of Waterloo. Wumpus allows control of the text unit retrieved based on structural constraints in the query.
A search engine designed for indexing database content. It natively supports MySQL, PostgreSQL, and XML pipe interfaces. It is written in C++ and has a GPL license.
An LGPL 2.1, open-source, fulltext search engine and column store written in C. Works with MySQL and Postgres. Site provides online documentation and downloads.
A GPLv3 search engine and crawler for urls, databases, and file systems. Comes with an XML/HTTP API, PHP/ASP client. Based on Apache Tomcat, Java Server Faces and JBoss RichFaces.
Java-based Apache licensed enterprise web crawler running on any platform, and integrating with virtually any search engines (open-source or commercial).
A packaged, Apache v2-licensed, enterprise search solution that leverages ManifoldCF for data sources, Solr for the search engine, and Cassandra for user management.