CompleteNewbiesClickHere for some ideas on how to get started.
New Kernel Hacking HOWTO (right now under-development on kernelnewbies)
CareerAdvice for people who want to get a job hacking the kernel
From basic to advanced Linux Kernel articles (people keep on updating this blog) http://whatisthekernel.blogspot.com/
Some existing documents on kernelnewbies site can be found here
KernelHackingTools to edit/navigate/hack the kernel source
mkkernel helps beginners.
testing new kernels and recovering...
SMPSynchronisation an intro on SMP locking
What to expect with Linux 2.5 (if you get /null appended to the URL, go back & try again (a few times if necessary))
UpstreamMerge How to submit your code to the upstream kernel.
WhyReiser4IsNotIn Why Reiser 4 is not in the upstream kernel.
PatchOfTheMonth Noteworthy and/or amusing patch submissions and replies.