Couchbase is described as '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' and is an app in the Development category. There are more than 50 alternatives to Couchbase for a variety of platforms, including Linux, Windows, Mac, Online / Web-based and BSD. The best alternative is PostgreSQL, which is both free and Open Source. Other great apps like Couchbase are SQLite (Free, Open Source), MongoDB (Free), MySQL Community Edition (Free, Open Source) and MariaDB (Free, Open Source).
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.
HSQLDB (Hyper Structured Query Language Database) is a relational database management system written in Java. It has a JDBC driver and supports a large subset of SQL-92 and SQL:2008 standards. It offers a fast, small (around 1300 kilobytes in version 2.
DB2 offers industry leading performance, scale, and reliability on your choice of platform from Linux to z/OS. DB2 supports storing XML and relational data and offers SQL PL and PL/SQL procedural languages.