For MIPSeb, there's Rough Cuts Linux, previously known as Hard Hat Linux, which is most of RedHat Linux 5.1 ported for MIPSeb. You can get this at ftp://oss.sgi.com/pub/linux/mips/redhat.
It is bundled along with M68k, UltraSparc, and PowerPC in a package called "Rough Cuts" pressed by RedHat, and available wherever RedHat products are sold. This is a very convenient way to get it without having to download 280MB. Redhat is no longer offering this product but if you're lucky you may still find the CD in some shops or somewhere on the net.
A distribution based on Red Hat 5.2 that's targeting the Cobalt Qubes. Those binaries will work perfectly on other MIPSel architectures and are available at ftp://intel.cleveland.lug.net/pub/Mipsel. ftp://bolug.uni-bonn.de/mips has various rpm packages from Redhat 6.0, 6.1 and 6.2.
A Debian port is underway. Current efforts are being bootstrapped using SGI/Linux as a base, and dpkg compiles natively with few changes. In addition to the SGI version, some interest has been shown in little endian platforms. Keep an eye on the Debian-MIPS Port page, http://www.debian.org/ports/mips/ for developments.
This distribution is for big-endian systems only so far. It's highly experimental, intended for developers' use in testing the latest versions of gcc, binutils, glibc, and the kernel. This is the only glibc 2.2-based distribution available for MIPS. You can always get the latest version of this distribution and accompanying release notes at ftp://oss.sgi.com/pub/linux/mips/mips-linux/simple. Also available is a cross-compiler system to aid in development.
Kevin Kissel of MIPS, Inc. has put together a compilation of the RedHat 5.2 mipsel distribution that includes the all the packages from the Cleveland LUG site ftp://intel.cleveland.lug.net/pub/Mipsel, plus quite a few others from elsewhere that are either newer than, or missing from, the Cleveland LUG distribution. This compilation can be found via HTML at http://www.paralogos.com/mipslinux or directly by FTP at ftp://ftp.paralogos.com/pub/linux/mips/RPMS/mipsel.