I was looking for lightweight free database for my application development. Here are the list lightweight open source or free databases available and all these are free to use.

1) SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. The source code for SQLite is in the public domain. http://www.sqlite.org/

2) HSQLDB (HyperSQL DataBase) is the leading SQL relational database engine written in Java. It has a JDBC driver and supports nearly full ANSI-92 SQL (BNF format) plus many SQL:2008 enhancements. It offers a small, fast multithreaded and transactional database engine which offers in-memory and disk-based tables and supports embedded and server modes. Additionally, it includes tools such as a command line SQL tool and GUI query tools.

The product is currently being used as a database and persistence engine in many Open Source Software projects and in commercial projects and products. In its current supported versions it is extremely stable and reliable. It is best known for its small size, ability to execute completely in memory, its flexibility and speed.

This feature-packed software is completely free to use and distribute under our licenses , based on the standard BSD license. Completely free of cost or restrictions and fully compatible with all major open source licenses. Java source code and extensive documentation included.

Our group was formed in 2001 to continue the closed Hypersonic SQL Project. We have actively developed and released 9 new versions of the database since April 2001. Current code has little in common with the Hypersonic SQL original. The latest version features the most extensive set of SQL features ever in an open-srouce database engine. The project enjoys a high ranking among all SourceForge projects, over 1,200,000 direct downloads, and hundreds of millions of copies distributed as part of other software packages. http://hsqldb.org/

3) Firebird is a relational database offering many ANSI SQL standard 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. It has been used in production systems, under a variety of names, since 1981. http://www.firebirdsql.org/