General

• Linux Documentation Project, Linux Headquarters
• Linux Journal, Linux Magazine, Byte Magazine: Serving with Linux

Processors

• Pentium 4 Processor Manuals

Linux Kernel

• Linux Kernel Archives, Linux Source Navigator
• Linux Kernel 2.4 Internals
• Linux-MM, Linux VM2.4 Overview, FreeBSD VM, Joe Knapka's 2.4 MM, Rik van Riel's zone-based memory allocator, Linux VM by Paul Wilson (yes, right. it's him :)
• comp.os.linux.development.system, Linux-kernel mailing list (faq), Kernel Traffic
• KERNELNewbies

Device Drivers

• Linux Device Drivers, 2nd Edition: on-line book

Application programming

• Linux Programmer's Guide

Multimedia

• Open Sound System Programmer's Guide
• OpenDVD.org