U1DB is a database API for synchronised databases of JSON documents. It’s simple to use in applications, and allows apps to store documents and synchronise them between machines and devices. U1DB itself is not a database: instead, it’s an API which can be backed by any database for storage. This means that you can use u1db on different platforms, from different languages, and backed on to different databases, and sync between all of them.
Storm is an object-relational mapper (ORM) for Python developed at Canonical.
Kumofs is a simple and fast distributed key-value store. You can use a memcached client library to set, get, CAS or delete values from/into kumofs. Backend storage is Tokyo Cabinet and it will give you great performance.
DumpTruck is a document-like interface to a SQLite database.