Elasticsearch is a search server based on Lucene and has an advanced distributed model. You can host the opensourced code yourself, on EC2 or use a service such as Bonsai, Found
Solr is an open-source search server based on the Lucene Java search library. You can host the opensourced code yourself, on EC2 or use a service such as WebSolr or SolrHQ.
Algolia is a hosted search platform.
Swiftype offers a fast, modern search that is easy to set up and use. Main feature include autocomplete, query result management, and analytics. There are free and paid (starting...
Denote Site Search
A powerful & smart search that is designed for applications and websites.