Apache Solr

Solr (or Apache Solr) is an open source enterprise search platform. 

Solr is derived from the Apache Lucene project.

Its major features include
 * powerful full-text search,
 * hit highlighting,
 * faceted search,
 * dynamic clustering,
 * database integration, and
 * rich document (e.g., Word, PDF) handling.

Providing distributed search and index replication, Solr is highly scalable.

Used on Projects
Multisite