There have been some people that have taken components for certain Macs (like logic boards, processors, and stuff from other systems) and have built their own "FrankenMac", if you will. However, most of these are hacks. Like most Tier 1 computer manufacturers (Dell, HP, Apple, etc), brand-name computers usually are designed to fit into a specific case design and not meant to be put in "white box" cases.
There are companies that make PPC motherboards, but they aren't as prevalent as the x86 component manufacturers are. For instance,
Genesi makes PowerPC based computers for use with their MorphOS operating system as well as various Linux/PPC distributions. Take a look at their prices though, and you'll see why they aren't found as often. They are cheap considering you are getting everything in one shot (motherboard, CPU, operating system, etc.), but still this is one of the few companies that is doing this.
As for the components that work with current Macs and such non-Apple PPC systems, check out the specs for the Pegasos on Genesi's page and the specs on the Apple machines on Apple's page and on
Low End Mac. You'll notice that they use standard PC parts as well.