Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Apache Solr - Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and...
ClassicPress - The WordPress fork. No Gutenberg. Great future!
ElasticSearch - Elasticsearch is an open source, distributed, RESTful search engine.
Textpattern - Textpattern is an elegant content management system that is free, open source software.
Algolia - Algolia's Search API makes it easy to deliver a great search experience in your apps & websites. Algolia Search provides hosted full-text, numerical, faceted and geolocalized search.