One of the most trusted and a highly reliable Firewall is CSF or ConfigServer Security and Firewall. This can be easily integrated with WHM/cPanel and is widely preferred by hosting providers to protect the Linux hosting servers.
Hurry up! ZoneAlarm is giving away the 2009 Pro version. ZoneAlarm Pro adds powerful, multi-layered security and additional protection. Includes Operating System Firewall, Network and Program Firewall, Anti-Spyware, Identity Theft Protection. The latest release improves the Operating System Firewall and applies more robust spyware detection and removal functions.