1. Nmap -It maps network and ports with the number one port scanning tool.
2. OpenVAS - open source vulnerability scanning suite manage all aspects of a security vulnerability management system from web based dashboards.
3. OSSEC - It is a host based intrusion detection system or HIDS, easy to setup and configure.
4. Security Onion - It is a network security monitoring distribution that can replace expensive commercial grey boxes with blinking lights.
5. Metasploit Framework - It tests all aspects of security with an offensive focus.
6. OpenSSH - It secures all traffic between two points by tunneling insecure protocols through an SSH tunnel.
7. Wireshark - It views traffic in as much detail as the user wants.
8. BackTrack - It is an Ubuntu based Linux distribution that is configured with hundreds of security testing tools and scripts.
9. Nikto - It is a web server testing tool and is used for firing at a web server to find known vulnerable scripts, configuration mistakes and related security problems.
10. Truecrypt - Truecrypt is a strong encryption utility that can encrypt entire volumes or create an encrypted container within a file system.