Kali Linux is not the only best available open source penetration testing and ethical hacking distro there are dozens. Kali Linux is, just like other Linux platforms, an open source project, but is maintained and funded by Offensive Security, which provides a handful of security and penetration testing services. Yes, it is yet another great distribution of Linux, which is targeted to the penetration testers and white hat hackers. If you are planning to start your career as an ethical hacker, or have interest in hacking, you must have heard about Kali Linux.
