If you're using custom hardware, you'll need a ROM monitor to help bring it up. A monitor generally provides a simple command prompt, with options for reading/writing memory and perhaps some power-on testing facilities which are useful before the kernel boots.
ROM monitors invariably include boot loader functionality, but if your device goes into production you may want a standalone boot loader or a stripped-down ROM monitor. There are several available on the net:
http://ppcboot.sourceforge.net/
This is an actively developed ROM monitor and Linux boot loader ideal for custom boards, derived from 8xxROM and FADSROM. It includes support for BOOTP, RARP and TFTP image downloading and booting.
http://www.thinsys.com/limon.html/
The Linux Monitor (LiMon) is a low level system start utility for the SBC8260. It also includes BOOTP and TFTP support.
http://pmon.groupbsd.org/
The PMON/2000 BootROM Monitor (PMON/2000) permits full-featured debugging to be performed on a target board that has a serial port, 512KB of ROM, and 128KB of RAM that can be dedicated to the Prom Monitor software.
http://ppcforth.sourceforge.net/
It is currently targeted for the IBM 403, 8xx is a goal for the future. It is rom-able so you can boot directly from it, and has some ifdef's to slim the code down to as little as 10K or as large as 30K. It accepts S records over the serial port for loading other software. Unfortunately it is written in assembly, but then it's really small and is released under the GPL. It is a small implementation of FORTH which is handy for testing and small programs, although the author went overboard and included multitasking and a lot of low-level device control words.
http://www.mot.com/SPS/PowerPC/teksupport/tools/DINK32/VERSION12/dinkindex.htm
http://www.s4l.de/powerpc.html
Derived from FADSROM. No longer actively developed, and only really of historical interest as it's superceeded by ppcboot.
An old ROM monitor and loader written for the FADS board: http://lists.linuxppc.org/listarcs/linuxppc-embedded/199912/msg00045.html
It's now superceeded by ppcboot.