Kali Linux

Kali Linux is the direct successor to BackTrack Linux and has become the default OS for penetration testers, red teams, and security certification courses (Offensive Security's own OSCP among them), shipping with tools like Nmap, Metasploit, Wireshark, Burp Suite, Aircrack-ng, and hundreds of others ready to use out of the box. It's built on Debian Testing and offers specialized build variants -- Kali NetHunter for Android devices, Kali ARM images for Raspberry Pi and similar boards, and pre-built VM/cloud images -- alongside the standard desktop ISO. As with any penetration-testing toolkit, its tools are intended strictly for systems you own or are explicitly authorized to test; running them against systems without authorization is illegal in most jurisdictions.