Apache Solr
Distributed search engine built on Apache Lucene — Yonik Seeley (CNET), 2004. Donated to ASF in 2006. The first large-scale "search server" before Elasticsearch.
XML/JSON configuration, RESTful APIs, distributed sharding (SolrCloud). Strong for "classic" enterprise search with facets, complex queries, custom relevance tuning. Built-in admin UI (Solr Admin).
Lost mainstream to Elasticsearch (simpler UX, faster release cycle). Lives on in e-commerce (Lucene stacks like Bloomreach, Magento), legacy enterprise search, academic search engines.