We have been using Solr on it's own as a database for years. Many times I hear the "don't use solr as a database" rhetoric but honestly for our usage it is amazing. We're using it as a document store and querying using fulltext and key/value indexes. It is fast and reliable and has handled millions of documents pretty easily. I've been happy with it.

Currently evaluating ElasticSearch and it looks really promising also. We may move over to it as it seems to have a bit more momentum and better support for cloud/scaling.

ElasticSearch and Solr are both search engines built on top of Lucene. ElasticSearch just has all the hipster hype these days. Maybe update the description to include all three.