Gentoo Linux Handbook
Gentoo is a free operating system based on either Linux or FreeBSD that can be automatically optimized and customized for just about any application or need. Thanks to a technology called Portage, Gentoo can become an ideal secure server, development workstation, professional desktop, gaming system, embedded solution or something else -- whatever you need it to be.
It is important to choose the correct architecture before proceeding with the associated Installation section of a Handbook. Be sure to verify the CPU's architecture before moving onward.
A note for new Gentoo users: if the CPU is less than five years old and the manufacturer is either Intel or AMD, choosing the AMD64 Handbook is probably the correct route