FleetDB is described as 'offers a flexible and expressive data model designed for the needs of modern application developers; a combination of schema-free records, declarative queries, automatically maintained indexes, and an optimizing query planner make it easy to manipulate and retrieve data'. There are more than 10 alternatives to FleetDB for a variety of platforms, including Mac, Windows, Linux, BSD and Online / Web-based. The best alternative is MongoDB, which is free. Other great apps like FleetDB are CouchDB (Free, Open Source), RethinkDB (Free, Open Source), CockroachDB (Free) and Titan Database (Free, Open Source).
MongoDB is a document database with the scalability and flexibility that you want with the querying and indexing that you need MongoDB stores data in flexible, JSON-like documents, meaning fields can vary from document to document and data structure can be changed over...
Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP / JSON API. It includes incremental map / reduce mechanics for queries and keeps data safe with an append-only datastore.
RethinkDB pushes JSON to your apps in realtime. When your app polls for data, it becomes slow, unscalable, and cumbersome to maintain. RethinkDB is the open-source, scalable database that makes building realtime apps dramatically easier.
Cockroach is a distributed key/value datastore which supports ACID transactional semantics and versioned values as first-class features. The primary design goal is global consistency and survivability, hence the name.
Couchbase is the NoSQL database market share leader, with production deployments at AOL, Deutsche Post, NTT Docomo, Salesforce.com, Turner Broadcasting Systems, Zynga and hundreds of other organizations worldwide.