Open Source, Distributed, RESTful Search Engine: ElasticSearch
ElasticSearch is an open source, distributed RESTful search engine built for the cloud on top of Lucene. ElasticSearch is schema less, just toss it a typed JSON document and it will automatically index it.
Search can be executed either using a simple, Lucene based query string or using an extensive JSON based search query DSL. Search though does not end with just queries, facets, highlighting, custom scripts, and more are all there to be used when needed.