Solr

Solr is an overlay of Lucene that adds functionalities and simplifies the roll-out of certain Lucene functions known to be excessively technical. Its development was initiated by CNET Networks, which decided to publish their work in 2006.

Solr is a corporate search server that centralises indexing and results serving operations. Solr can communicate with other applications via many protocols based on open standards. It also offers an admin interface in web mode. One of Lucene’s major characteristics is its capacity to index the content by field or by attribute, meaning that a document is not analysed as a simple set of words; rather it is made of fields, with each field being a string of words (terms). Solr makes it possible to take full advantage of this functionality. This allows for much more detailed management of relevance and advanced searches.  
Also noteworthy is the improved distribution of processing results and data thanks to the Solr Cloud functionalities included in version 4.0.
Solr is distributed under the Apache License.