Linux kernel development
[https://kernelnewbies.org/ kernelnewbies.org] the main kernelnewbies website
[http://www.linuxfromscratch.org/ Linux From Scratch] detailed instructions on how to build your own Linux system entirely from source code
[http://www.linuxfromscratch.org/hints/downloads/files/essential_prereading.txt Linux From Scratch - Essential Pre-Reading] more help for Linux From Scratch
[http://kernelbook.sourceforge.net/wiki/?KernelWiki KernelWiki] (appears to be broken at the moment)
LinuxMM Linux memory management wiki
[http://linux-mm.org/HugePages] Huge pages wiki
[http://linuxfs.pbwiki.com/] Filesystems wiki
[http://linux-net.osdl.org/ Linux-Net] Linux networking wiki
[http://lkml.org lkml.org] linux-kernel mailing list
[http://kernel.org kernel.org] the home of the Linux kernel
[http://kernel.org/doc kernel.org/doc] various documentation around the kernel
[http://linux-news.net] linux distro news.
General Linux
[http://tldp.org/LDP/abs/html/index.html Advanced Bash Scripting Guide]
[http://www.regular-expressions.info/reference.html Regular Expressions Reference - Basic Syntax]
[http://www.grymoire.com/Unix/Sed.html Sed - An Introduction and Tutorial]
[http://www.pendrivelinux.com/ Boot and run Linux from a USB flash drive]
[http://www.linuxlinks.com/ Linux Links]
[http://freshmeat.net/ Freshmeat]
[http://www.linuxcookbook.com/ Linux Cookbook - tips and hints]
[http://www.gNewSense.org/ gNewSense] a distro based on Ubuntu with all the non-free parts removed
[http://www.freesoftwaremagazine.com/ Free Software Magazine]
[http://www.eiv.com/vim-cook.html Vim Cookbook]
[http://thomer.com/vi/vi.html Vi Lovers Home Page]
[http://vimdoc.sourceforge.net/htmldoc/version7.html Vim Documentation]
Security related
[http://www.linuxsecurity.com/ Linux Security] detailed Linux security news site
News sites
[http://www.lwn.net Linux Weekly News] - follow the kernel link (changes weekly) (the current kernel page is always http://lwn.net/current/kernel.php3).
[http://www.kerneltrap.org/ Kerneltrap] - news on various free OS kernels
[http://web.archive.org/web/20071012002855/http://kerneltraffic.org/ Kernel Traffic (link to Internet Archive)] - a digest of what was discussed on the Linux-kernel mailing list (original site has disappeared)
Other operating systems
[http://www.dragonflybsd.org/docs/articles/vkernel/vkernel.shtml A Peek at the DragonFly Virtual Kernel]
[http://wiki.dragonflybsd.org/index.cgi/RecentChanges DragonFly wiki - Recent Changes]
[http://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs Plan 9 overview on Wikipedia]
[http://www.osnews.com/story/15235/Investigating-the-Plan-9-Operating-System/ Investigating the Plan 9 Operating System (article)]
[http://www.minix3.org/ The MINIX 3 Operating System]