LiteDB
An open source MongoDB-like database with zero configuration
..., single datafile (like SQLite), recovery data in writing failure (WAL mode), map your POCO class to BsonDocument, fluent API for custom mapping. Cross collections references (DbRef), store files and stream data (like GridFS in MongoDB), LINQ query support, free for everyone, including commercial use. LiteDB supports SQL-like language for data and structure manipulations. You can insert, update, delete or query your database using a very similar SQL relational language.