Free portable database [closed]

Answer 1

Firebird embedded

About:

Firebird is an open source relational database offering many ANSI SQL-99 features that runs on Linux, Windows, and a variety of Unix platforms. Firebird offers excellent concurrency, high performance, powerful language support for stored procedures and triggers.

Answer 2

If you're working in Java, HSQLDB might be a good fit. Otherwise, SQLite.