https://github.com/adewes/blitzdb
My motivation was that I needed (wanted?) a pure-Python document database that does not have any dependencies (like pymongo) and provides querying capabilities similar to MongoDB.
Currently, Blitz is under active development and comes with a file-based backend as well as a MongoDB backend.
Contributions to the codebase and feedback are highly welcome :)
The issue tracker contains various suggestions for contributions, with various difficulty levels: