Hardening operating system is important to protect your environment against any compromise. one of the open source tools that you can use for hardening Unix and Linux based systems is Lynis. Lynis will run several hundreds of tests and perform an audit for your system so it will check the configuration files to find out if you have the correct configuration and report for what are the gaps you have on your systems.
Lynis help you with taking the right measures and check the related controls and define your improvement plan this to meet security standards such as Basel II,GLBA,HIPAA , ISO27001/ISO27002, PCI-DSS and SOx (Sarbanes-Oxley). Lynis will make the compliance scan you need to evaluate your system against the standards so you can have a checklist with the action plan to properly harden your system.
At the moment there are an open source version that you can use for security auditing,vulnerability scanning and system hardening. While you can find an enterprise version which adds more components for the compliance check and security. you can download Lynis over this link: http://cisofy.com/products/