CHRP is an acronym for Common Hardware Reference Platform, sometimes pronounced "chirp." It was developed by Apple, Motorola, and IBM in the mid-90's. The idea was to create an open platform providing software compatibility for multiple operating environments. It was seen as the next stage of evolution for the PC platform, replacing IA32-based CISC microprocessors with the PowerPC RISC architecture, and the obsolete PC BIOS with Open Firmware. It also made use of the OpenPIC interrupt standard that eliminated the hassle of IRQ conflicts. There were several operating systems developed for the CHRP including MacOS, Windows NT, AIX, BeOS, and Linux.
The CHRP was never widely adopted. There were numerous prototypes and reference machines built, and a few reached production. Apple's decision to kill the clones, and to remove CHRP support in their operating system helped bring about its downfall. Microsoft's termination of CHRP versions of Windows NT contributed to it as well. In fact, the only two publicly available operating systems known to run on the CHRP are Linux and AIX.
Computers based on CHRP:
Open Desktop Workstation: The only PowerPC-based desktop solution endorsed by both IBM and Motorola, based on the Pegasos motherboards.
Pegasos I and II: Manufactured by Genesi. The Pegasos II is a Micro ATX form-factor motherboard supporting either G3 or G4 microprocessors. it uses Open Firmware and is supported by several operating systems such as MorphOS, Linux, and OpenBSD. These boards have been quite successful, as Genesi hasn't been able to keep up with consumer demands. More information at http://www.pegasosppc.com/.
Umax Power 6000: A CHRP technology demo. Pictures and brief information at: http://www.macinfo.de/rechner/clone-umax.html
PIOS ONE - Maxxtrem, transAM: I couldn't confirm if these ever reached production stages. Even so, they would be a rare find.
MaxxBox 930: Sold in Germany. These came in neat looking cube style cases. Available with 233, 266, and 333 megahertz PPC 604e CPUs. http://www.everymac.com/systems/maxxboxx/
References
(1) "Common Hardware Reference Platform (CHRP) Documentation." http://www.firmworks.com/www/chrpdoc.htm
(2) "Convergence: Common Hardware Reference Platform." http://www.convergence.org/platforms/chrp/main.html