Search API. This module provides a framework for easily creating searches on any entity known to Drupal, using any kind of search engine.
For site administrators, it is a great alternative to other search solutions, since it already incorporates facetting support and the ability to use the Views module for displaying search results, filters, etc. Also, with the Apache Solr integration, a high-performance search engine is available for this module.
Developers, on the other hand, will be impressed by the large flexibility and numerous ways of extension the module provides. Hence, the growing number of additional contrib modules, providing additional functionality or helping users customize some aspects of the search process. The prime example of a site using this module is drupal.org itself – the Search API, along with the database backend and Views, is used to generate the issue queue listings. Apache Solr Search Integration.
Facet API.