giant PS2, PS3 Enterprise Edition, and "just a Power Macintosh G5"
Also known as the "Card Cage", this is essentially a 6502-based modular-bus computer (left) and a peripheral chassis for the PPU (right), with all the components of the APU (for sound, in the left assembly), PPU (for graphics, the entire right assembly) and the memory and other control and I/O logic, out on discrete components on cards that slot into the chassis.
It would be an interesting project to build one of these from scratch!