Getting the kernel
The canonical Linux kernel source is maintained in a git repository hosted on kernel.org, and this is what the vast majority of people should use. If you're unsure what kernel source to use then in all likelihood you should take the latest stable release tagged in the git repository at git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git.
Linux-mips.org hosts various kernel git repositories, primarily containing work in progress which will eventually be merged into the kernel.org repository described above. You can browse these git repositories using cgit.
In general users are likely to be best served by the latest stable release. If you have a particular need for a single kernel version with a long support period then one of the longterm stable kernels may better suit your needs.
Developers submitting patches to linux-mips should base their work on either the latest stable or -rc tag, or upon the current mips-next or mips-fixes branch as appropriate at the time of their submission. The mips-next & mips-fixes branches hold work queued for submission in the next kernel merge window or the currently in-development kernel version respectively. They are hosted at git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git, and browseable using cgit.
MIPS maintains an additional repository of "engineering kernels" which include patches that haven't yet made it into mainline. These are typically focused on support for their synthesizable cores and their Boston, Malta and SEAD-3 evaluation & development boards, but also often include other fixes & architectural work.
These engineering kernels can be found as tags named "eng-<YYYYmmddHHMM>" in the linux-mti.git repository, which you can clone or fetch from git://git.linux-mips.org/pub/scm/linux-mti.git. The "eng" branch will always reflect the latest tag. Please note that this is rebased often, so if you wish to work atop it & stay up to date you should be prepared to rebase too.
Support for the recently announced nanoMIPS ISA is not yet in mainline, but initial support can be found in the nanomips-v4.15 branch of the MIPS Technologies linux-mti.git repository described above.
Linux-mips.org has switched from CVS to git for the kernel tree. Therefore the CVS tree is frozen that is there will not be any changes to that tree anymore, not even bug fixes. The anonymous CVS server is still running so users with a checked out copy can continue using cvs diff etc. Only the linux-2.2, linux-2.4 and HEAD (2.6) branches are in this CVS repository. You can browse the old CVS tree in CVSweb.