Database with a JDBC driver, or use the embedded pure Java database.
Java 5: because Openfire is a pure Java application, it will run on any platform where Java 5 (JRE 1.5 or better) is available. Java 6 is recommended.
Software & Hardware Options
Windows 2000, Windows XP, Windows Vista, Windows Server 2003, Windows Server 2008
Linux, Solaris, or other Unix with Java 5 support
Mac OS X
MySQL (5.x or better recommended)
Oracle (10g or better recommended)
Postgres (7.x or better recommended)
IBM DB2 (v7 or better recommended)
SQL Server (2000 or better recommended)
HSQL DB (also used as the embedded database)
Any other database that has a robust JDBC 2.0 compliant driver
At least 384 MB of available RAM and one 1.5 GHz processor.
At least 768 MB of available RAM and one 3.0 GHz processor.
At least 1.5 GB of available RAM, two 3 GHz processors, and one connection manager on the same machine.
At least 2.0 GB of available RAM, two 3 GHz processors, and between 1 and 4based on exact size, each on their own equivalent machine.
We recommend a backup solution for server-side applications. At a minimum you should back up your database on
a regular basis as well as the configuration files for Openfire (note, these are stored in one directory).
A backup solution will help you recover from hardware failures or human error (it's been known to happen!).
Original source: http://www.jivesoftware.com/products/openfire/resources/requirements.jsp (No longer online. Jive Software is the parent company of igniterealtime.org)