The open source Lucene search engine library is used by quite a few websites, including Twitter and Facebook, though few people know it.
Apache, which shepherds the project, has just released the latest major version of both the Lucene library and the Solr search platform built on top of it.
Lucene and Solr 4.0 are now considered stable and are available for download. The big update in this latest release has to do with scalability, the library and search engine should be able to make better use of a cluster of servers.
The software has been in beta since August as the final bugs and issues were ironed out. Lucene is a Java library that provides the search functions.
Solr provides an API to access the search features built into Lucene, abstracting some of the work needed to put together a custom search engine.
Apache Lucene 4.0 is available for download here.
Apache Solr 4.0 is available for download here.