collective.solr integrates the Plone CMS with the Solr search engine.
Apache Solr is based on Lucene and is the enterprise open source search engine. It powers the search of sites like Twitter, the Apple and iTunes Stores, Wikipedia, Netflix and many more.
Solr does not only scale to any level of content, but provides rich search functionality, like faceting, geospatial search, suggestions, spelling corrections, indexing of binary formats and a whole variety of powerful tools to configure custom search solutions. It has integrated clustering and load-balancing to provide a high level of robustness.