1) Mysql:- Most popular open source database because of its consistent fast performance, high reliability and ease of use.
MySQL runs on more than 20 platforms including Linux, Windows, OS/X, HP-UX, AIX, Netware, giving you the kind of flexibility that puts you in control. Url:- http://www.mysql.com/
2) Postgresql:- PostgreSQL is free to modify to fit your particular needs. POSTGRES used the PostQuel query language, PostgreSQL uses a subset of SQL. Url:- http://Postgresql.org
3) Mssql:- MSSQL Server is a comprehensive database software platform providing enterprise-class data management and integrated business intelligence (BI) tools. An integrated suite of management tools and management application programming interfaces (APIs) to provide ease of use, manageability, and support for operating large-scale SQL Server deployments. http://www.microsoft.com/sql/downloads/default.mspx