We're using ElasticSearch [1] and finding it really nice to work with.<p>It's basically Lucene with a nice JSON/RESTful interface and distributed out of the box.<p>Unlike most other Lucene based systems that I've played with it allows you to get started without having to define your schema upfront, so you can just chuck data at it and worry about field types later.<p>It's under constant development and the community is really helpful.<p>[1] - <a href="http://www.elasticsearch.com/" rel="nofollow">http://www.elasticsearch.com/</a>