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 (linked below), providing additional functionality or helping users customize some aspects of the search process.
There is a screencast available, showing off some of this project's features. (Note, however, that it doesn't represent the current development state – e.g., facets can now be used with the database search, too.)
Note on security
In general, the Search API doesn't provide any kind of access restrictions (as this cannot be done on a generic level). It is therefore your responsibility to take care that only accessible items are indexed or results displayed (e.g., by using appropriate filters).
There is, however, built-in support for node access checks via the Node access data alteration (q.v. for details). Also, the Search views module contains an option to do additional access checks on all entities in search views.
At the moment, this project contains, apart from the core API module, the following extension modules:
This module integrates the Search API with the Views module, allowing searches on any index to be created and viewed via Views. All of an entity's properties, as well as those of related entities (e.g. a node's author's name), are available as fields, filters and arguments for all indexed fields are available and sorts (as well as click sorts) can be created on any indexed single-valued field. Also some additional features, like linking the results to the entity, are available.
This module provides the ability to create facets on any search executed anywhere in Drupal, be it a search page, a view or any other source. The feature isn't available for all servers, though, since it would be too complex to implement for some service classes. Right now only the Solr servers support this, facet support for the database-based searches might be done in the future.
Other available backends and extensions
You can find a list of available extension modules in the handbook.
You will have to download and enable the Entity API module to use the Search API.
To use the Views integration, you'll (of course) also need the Views module.
Furthermore, you'll need to enable at least one module that provides a service class.
Information for developers
Extensive documentation for other developers, who might want to extend the framework's capabilities themselves, is available. If you are trying to implement some extension point and are stuck, please don't hesitate to ask in the module's issue queue. Please also add your module to the list of extension modules (linked above)
All developers working in the Search API ecosystem are advised to keep an eye on #1188562: [meta] Important project announcements for important updates. (Could also contain useful information for site builders.)
Also interesting for developers might be a session I held at Drupal Dev Days Brussels, which has the slides available online and explains the framework's architecture from a developer's point of view.
AJAXified search facets with Views and Panels
Discussions about performance (compared to apachesolr)
How to setup Search API with Apache Solr
Adding custom fields to a search index
My session from Drupalcon Munich with a setup tutorial
If you are interested in sponsoring a particular feature or new related module, please contact me. I'm also available for consulting.