Solr

Apache Solr is a widely used, lightning-fast, open-source enterprise search platform based on Apache Lucene. It is aimed at users who are looking for a reliable, scalable, and fault-tolerant solution. Solr offers distributed indexing, replication and load-balanced queries, automatic failover and recovery, centralized configuration, and more. It is used by many of the world's largest internet sites. The platform offers hundreds of features that make Solr incredibly versatile. There are no specific price details, as it is an open-source software.