strus
The open source (GPL v3) project strus provides a collection of programs and libraries (language C++) for building a competitive scalable full-text search engine. The search index is distributable and user rights can be implemented.
Sphinx (search engine)
Sphinx is a free software search engine designed with indexing database content in mind. It currently supports MySQL, PostgreS ...
Cludo Site Search
Cludo is a powerful search solution to your website. It is built on top of Elasticsearch which means you do not have to do any ...
Apache Solr
Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and JSON APIs. Its major feature ...
Xapian
Xapian is an open source probabilistic information retrieval library, released under the GNU General Public License (GPL). It ...