Further information about kernel programming, especially regarding portable modules and networking, can be found at the following sites.

General

"Linux Device Drivers, 2nd Edition": Online Book O'Reilly's book. Worth its weight in gold.
LWN.net v2.4/v2.6 porting guide Everything you need to know to write portable kernelspace code
KernelNewbies.org Contains a host of useful information. Don't let the name put you off.
linux kernel cross reference complete cross references for various kernel versions. Use its identifier search instead of grepping your own sourcetree.
the linux kernel API (alternate link) reference manual for the kernel's equivalent of libc
"the linux kernel" an aptly named kernel overview
the linux kernel mailing-list FAQ read this before posting to the list

Modules

The Linux Kernel Module Programming Guide module centric documentation
module init tools FAQ information on porting modules to the v2.6 kbuild infrastructure

Networking

"HOWTO Add a New Network Protocol in LINUX" in case you need to hook into the protocol handling subsystem
BPF(4) man page for those of you that don't know, the linux socket filter is based on this: the Berkeley Packet Filter.
LinuxJournal: "Roll your own firewall with netfilter" a brief primer into netfilter programming
"HOWTO use netfilter hooks" more detailed information on netfilter programming
"The journey of a packet through the linux 2.4 network stack" a primer into the linux kernel network stack
"A Map of the Networking Code in Linux Kernel 2.4.20" you can't get more detailed than this.
"Network Buffers And Memory Management" by Alan Cox. Describes the main network datastructure (sk_buff) in great detail, especially regarding memory usage

Other Subsystems

the linux kernel procfs guide (alternate link) Information on using /proc for kernelspace/userspace communication
"Understanding The Linux Virtual Memory Manager" All you ever might need to know about linux VM management. Might be a bit outdated, as the VM changed a lot lately.

Do you know of other useful kernel programming links? Or have you found an error? Please post a message in our kernel programming forum (anonymous posting is allowed).