Linux Kernel Newbies, imho, is the best resource for who wants to start in linux kernel development.
Maybe you are interested to buy a book:
- Linux Kernel Development 3rd edition 2010;
- Understanding the linux kernel 3rd edition 2005;
- Linux Device Drivers 3rd edition 2005.
Free resources (only a few of them):
- Linux Kernel Hacking;
- Hacking the Linux 2.6 kernel, Part 1: Getting ready;
- Hacking the Linux 2.6 kernel, Part 2: Making your first hack;
- Kernel coverage at LWN.net;
- Unreliable Guide To Hacking The Linux Kernel;
- The newbie’s guide to hacking the Linux kernel;
- vger.kernel.org – list services for the linux kernel developers