Andrew Tetlaw at SitePoint - Watch out for CouchDB:

"A view is a dynamic structure that acts like a search query, providing a virtual table of documents matching the query. The query, previously expressed in a proprietary language, is now a JavaScript function which is used to determine which documents to include in the view. Because the views are completely virtual you can have as many as you like and you can add or remove them at any time without touching any of your data. Views are indexed and regularly updated to keep tabs with the state of the documents in the database.

CouchDB has some other impressive attributes; it’s fully ACID compliant, has a security model built in, bi-directional incremental replication and conflict resolution."