The PowerPC G5 processor is the latest and final family of PowerPC processors that will be used in Macs. In addition, it will also be used in the XBox 360 and presumably, Nintendo's next game console, code-named "Revolution."
The original G5, as can be found in most G5-based Power Macs and iMac G5's. Like their predecessor, they support the AltiVec instruction set. They do not include a third-level cache, because the front side bus speed, and therefore the main memory, is run at the same speed.
A low-power variant of the PPC970. While IBM developed this for the PowerBook G5, it has not (and presumably never will) make it to stores due to heat issues, and also becuase Apple has announced a switch to Intel processors.
A dual-core variant of the 970FX. This processor has not been released yet, but two of these will presumably be at the heart of the final Power Mac G5. Clock speeds are expected to finally reach 3 GHz, or at least come close.