The current version of the Linux kernel can be found on kernel.org and will tend to be somewhat ahead of the MIPS/Linux version (see CVS below) but behind in some MIPS-specific regards. Older and more stable versions of the kernel for MIPS are available for download - see the section on Distributions for locations.
For those who always want to stay on the bleeding edge, and want to avoid having to download patch files or full tarballs, we also have an anonymous CVS server. Using CVS, you can checkout the Linux/MIPS source tree with the following commands:
cvs -d :pserver:cvs@ftp.linux-mips.org:/home/cvs login (Only needed the first time you use anonymous CVS, the password is "cvs") cvs -d :pserver:cvs@ftp.linux-mips.org:/home/cvs co <repository>where you insert linux, libc, gdb or faq for <repository>.
There is a mailing list for information on what gets committed to this repository.
Via http://www.linux-mips.org/cvsweb, you have direct access to the new Linux/MIPS kernel sources, and a few other projects hosted in the same CVS archive. The intuitive interface allows you to follow the development at the click of your mouse.