We have had a good deal of luck using ElasticSearch to index and query our couchdb databases on some projects. Couch publishes a _changes feed that allows ES to subscribe to updates and index them as soon as they hit the database. It has a very powerful JSON based query language that eased a lot of the pain points related to couchdb views for us.

I wrote an article about my experiences with it : http://developmentseed.org/blog/2011/may/31/flexible-facetin...