Sphinx, manticore, solr, elastic, lucene, meilisearch, bleve, typesense, xapian, whoosh, yioop. Just a few. Keep in mind most are indexing engines and not full

Sphinx, manticore, solr, elastic, lucene, meilisearch, bleve, typesense, xapian, whoosh, yioop. Just a few. Keep in mind most are indexing engines and not full featured general purpose search engines. You have to build a lot on top to get something remotely close to google.