Security hardening and analysis tools.
Linux is a great operating system. Personally I use RHEL Linx types such as Fedora and Centos. Always go for the minimal installation packages. Only install the packages you need. Make sure to read the documentation, and check the configuration settings!
Lynis – Is a great tool that runs checks on your environment and gives you suggestions. It requires root privileges to be effective. Another option could be linux-checklist. Which works in a similar fashion.
Rkhunter – ‘RootKitHunter’ is a tool that scans your linux system for known rootkits and threats.