DovetailDB is a schemaless, JSON-based database. It is similar in spirit to Amazon's SimpleDB, but allows for javascript to be executed on the server, and, within that context, provides full transaction support.

DovetailDB's native JSON data format and embedding of a javascript environment makes it look like CouchDB, but DovetailDB manages the data indexing under the hood. (less developer effort, but less customizable)

It is also designed to integrate directly and easily with client-side Javascript, making it one of the fastest web application development platforms in existence. Server-side Javascript hooks may be written to enforce business rules and control access. DovetailDB supports Comet queries, allowing Javascript clients to be notified asynchronously when a query would have started to return data.

Try it out in your browser now.