The Apache Lucene project just released Apache Solr version 4.5. Please read the release announcement at http://lucene.apache.org/solr/solrnews.html#5-october-2013-apache-solr-45-available and see CHANGES.txt for all details.

Some highlights are multi-threaded faceting (SOLR-2548), multi-threaded updates with CloudSolrServer (SOLR-4816) and more intelligent routing of documents with SolrCloud.

Enjoy!