Linux From Scratch Guidebook
Pre-compiled Windows, Max, or Linux OS is great. They provide everything everybody needs to start their own digital life. But since they are made for everybody, they contain so many unnecessary packages and drivers, slowing down your computer. That's why a custom-compiled Linux is great. You get to choose your own drivers and softwares, effectively reducing system load and required space for the OS. This guidebook, is a very good start for everybody who is interested in building their own Linux OS.

Kali Linux, The most advanced penetration testing distribution, Ever
Pre-compiled Linux OS, made for web-penetration. Originally developed as 'BackTrack' the developers took a step further and turned it into a must-have-OS for many hackers and web-server testers. It contains over 300 penetration testing programs, supports multiple languages, and is free and always will be. Like any Linux OS, you can also choose to compile your own ISO image, or download the pre-compiled one. A very good start for anybody interested in Linux and web-security.