For MIPSeb, there's Rough Cuts Linux, previously known as Hard Hat Linux, which is most of Red Hat Linux 5.1 ported for MIPSeb. You can get this at ftp://ftp.linux.sgi.com/pub/hardhat.
It is also bundled along with M68k, UltraSparc and PowerPC in a package called "Rough Cuts" pressed by Red Hat, and available wherever Red Hat products are sold. This is a very convenient way to get it without having to download 280MB. You can order Rough Cuts directly from Red Hat at http://www.redhat.com/product.phtml/RC1000.
As well, there's a distribution based on Red Hat 5.2 that's targetting the Cobalt Qubes; those binaries will work perfectly on other MIPSel architectures available at ftp://intel.cleveland.lug.net/pub/Mipsel.
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.