From the [http://janitor.kernelnewbies.org Kernel Janitors website] : «We go through the linux kernel sources, doing code reviews, fixing up unmaintained code and doing other cleanups and API conversion. It is a good start to kernel hacking.»
If the site is down, there's a local copy of all the content at: [http://kernelnewbies.org/MigratingInProgress]
"The Kernel Janitors Project" is a good point to start for newbie kernel hackers. The project maintains a [http://kernelnewbies.org/KernelJanitors/Todo TODO list] from which contributors can pick a job which fits their skills. The tasks are mainly code cleanups so one can dive into kernel code without knowing all the technical details already. It's also a good point to learn about how to (or how not to) submit patches to the kernel.
The project keeps its own mailing list (http://vger.kernel.org/vger-lists.html#kernel-janitors) and the maintainer (Alexey Dobriyan at the moment) also irregularly produces a patchset which can be found on http://coderock.org/kj/.
UPDATE: There's a newer Kernel Janitors page at [http://code.google.com/p/kernel-janitors/], but it seems that some of the content from above hasn't been moved over, so you should look at both sites.