IBM DB2 on Linux. DB2 Universal Database (DB2 UDB) is a relational database management system. It supports and embraces open standards including Java and XML, and integrates with many open source products such as Apache, PHP, Perl, and Python. DB2 UDB is the most scalable database in production today that can manage mission-critical data on a single PC, SMPs, clusters, as well as the mainframe for Linux. If you want to know more try the following URL http://www-3.ibm.com/software/data/db2/linux/.
MySQL. This popular database has recently been purchased by Sun. The new release 6 includes many advanced options such as partitioning, stored procedures, transaction support etc.usually found only in commercial databases. Ypu will find useful information, downloads and tutorials at MySQL DevZone.
Oracle on Linux. If you want to install Oracle on your Linux box, some useful documentation can be found at the following URL http://www.puschitz.com/OracleOnLinux.shtml
Postgres. The open source database EnterpriseDB Postgres has a high degree of compatibility with Oracle and can be used as an alternative to Oracle in the ERP application Compiere ERP and CRM.