HSQLD is a mature, high-performant database written in Java. It is used in several high profile popular commercial and open source products like Open Office 2.0 (Office productivity suite; competitor of MS Office), JBoss (Application Server), JFox (Application Server), Jonas (Application Server), Mathemetica, Hibernate (ORM), Jira (Defect Tracking), TrackStudio (Defect Tracking), C-JDBC (Database Clustering) etc.
It is a lightweight, embeddable database which eliminates the challenge of separate installation and configuration for the database component as with other databases like Oracle, SQL Server or MySQL.