Solr

I've began to work more and more with the great “low-level” library Apache Lucene created by Doug Cutting. For those of you that may not know, Lucene is the indexing and searching library used by great entreprise search servers like Apache Solr and Elasticsearch.

Don't get me wrong, i love Apache Solr, i think it's a wonderful project and the versions 4.x are definitely something you should check out when building a proper search engine.