Alternative front ends
While it appears to the end-user as one CouchDB instance, it is in fact one or more nodes in an elastic cluster, acting in concert to store and retrieve documents, index and serve views, and serve CouchApps. BigCouch has been developed and is continually maintained by Cloudant who offer hosted CouchDB as a service.
Clusters behave according to concepts outlined in Amazon's Dynamo paper, namely that each node can accept requests, data is placed on partitions based on a consistent hashing algorithm, and quorum protocols are for read/write operations.
GT.M Database Engine with Extreme Scalability and Robustness
M/DB:X is a simple, lightweight, yet powerful hybrid JSON/Native XML Database
Enterprise Web Developer (EWD) is an advanced web application delvelopment technology and Ajax framework
PIP includes a JDBC driver and a SQL engine
This forum thread includes hints how to integrate PIP into an existing GT.M installation
gtm-rb is a first draft of a Ruby client library for the GT.M database engine through the M/Wire protocol
InterSystems Caché is an advanced but commercial 'successor' of GT.M
Data stored in Caché can be accessed using JDBC and ODBC.
Max Lapshin wrote a related Ruby driver
Although Lucene is a search index, and not a database, if your fields are reasonably small, you can ask Lucene to store them in the index.
Apache Lucene full-text search index
Apache Solr enterprise search platform
ActiveLucene is like ActiveRecord but with Lucene the full text search engine.
Sunspot Solr-powered search for Ruby objects
There is a bunch of Python and Ruby code to steal from:
Notes on MongoDB, Why I think Mongo is to Databases what Rails was to Frameworks, MySQL and MongoDB working together in Kanbanery thanks to the awesomeness of DataMapper
CarrierWave supports MongoMapper and offers compatibility to Paperclip
GridFS FUSE allows you to mount a MongoDB GridFS instance as a local filesystem
John Nunemaker's fork of Grip (from the author of MongoMapper)
Neo4j is a graph database. It is an embedded, disk-based, fully transactional Java persistence engine that stores data structured in graphs rather than in tables. A graph (mathematical lingo for a network) is a flexible data structure that allows a more agile and rapid style of development.
Neo4j.rb includes a Lucene wrapper.
InfoGrid is an Internet Graph Database with many additional software components that make the development of REST-ful web applications on a graph foundation easy.
Ontopia for building, maintaining, and deploying Topic Maps-based applications.
OpenLink Virtuoso Universal Server
is a scalable cross-platform server that combines SQL/RDF/XML Data Management with Web Application Server and Web Services Platform functionality.