Titan Database is described as 'Titan is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster' and is an app in the Development category. There are more than 50 alternatives to Titan Database for a variety of platforms, including Linux, Windows, Mac, Online / Web-based and BSD. The best alternative is DBeaver, which is both free and Open Source. Other great apps like Titan Database are PostgreSQL (Free, Open Source), SQLite (Free, Open Source), MongoDB (Free) and MySQL Community Edition (Free, Open Source).
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.
Firebird is a relational database offering many ANSI SQL-92 features that runs on Linux, Windows, and a variety of Unix platforms. Firebird offers excellent concurrency, high performance, and powerful language support for stored procedures and triggers.
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.
Hazelcast is an in-memory Open Source data grid based on Java. By having multiple nodes form a cluster, data is evenly distributed among the nodes. This allows for horizontal scalability both in terms of available storage space and processing power.