Linux kernel development
[http://kernelnewbies.org/ kernelnewbies.org] the main kernelnewbies website
[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://wiki.dragonflybsd.org/index.php/DragonFly_Status DragonFly Status] "Changelog" of DragonFly's kernel
[http://lkml.org lkml.org] linux-kernel mailing list
[http://kernel.org kernel.org] the home of the Linux kernel
[http://linux-news.net] linux distro news.
Other operating systems
[http://wiki.dragonflybsd.org/index.php/DragonFly_Status DragonFly Status] "Changelog" of DragonFly's kernel
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://kerneltraffic.org Kernel Traffic] - a digest of what was discussed on the Linux-kernel mailing list
[http://www.kerneltrap.org/ Kerneltrap] - news on various free OS kernels