Xapian!

Algolia provides a developer-friendly RESTful API for website and app instant search. Most web services and mobile apps, such as Spotify, Salesforce or Amazon need to provide a fast and meaningful access to database objects via a simple search box. People want to find songs, invoices, products in just a few keystrokes.

ElasticSearch is a distributed, RESTful, free/open source search server based on Java and under the Apache License ( Lucene ).

Solr is an open source enterprise search server based on Lucene search library, with XML/HTTP and JSON APIs. Its major features include powerful full-text search, hit highlighting, faceted search, dynamic clustering, database integration, and rich document handling. Providing distributed search and index replication, Solr is highly scalable.

OpenSearchServer is an open source application server allowing development of index-based applications like search engines. Main features of OpenSearchServer are : An integrated crawler for databases, web pages and rich documents; a user friendly GUI allowing development of most applications with some clicks through an interface built in Zkoss; snippets; faceting; an html renderer for integrating search results in a page; and monitoring and administration features.

The open source (GPL v3) project strus provides a collection of programs and libraries (language C++) for building a competitive scalable full-text search engine. The search index is distributable and user rights can be implemented.

Unlimited Searching

Cludo is a powerful search solution to your website. It is built on top of Elasticsearch which means you do not have to do any further developing, or hire any developer concerning site search. Cludo's focus is to deliver an easy to use search engine with the greatest support and tools.

SearchBlox is an out-of-the-box Enterprise Search Solution built on top of Apache Lucene/Solr. It is fast to deploy, easy to manage and available for both on-premise and cloud deployment.

Manticore Search is a feature-rich search engine that is blazing fast, reliable and scalable. It is 100% open source project started as a fork of Sphinx Search.

Expertrec site search engine helps you add ultra fast search to your website . It adds a superb autosuggest and search listing pages where people can find products with a few keystrokes. Along with this you get complete control over your search results with complete merchandising options and access to real time search analytics.

Azure Search makes it easy to add powerful and sophisticated search capabilities to your website or application. Quickly and easily tune search results and construct rich, fine-tuned ranking models to tie search results to business goals. Reliable throughput and storage provide fast search indexing and querying to support time-sensitive search scenarios.

The Google Mini is a smaller and lower-cost solution for small and medium-sized businesses to set up a search engine that allows them to index and search up to 300,000 documents.