Which version of Mac OS X was the last to support PowerPC processors?

Mac OS X Leopard, version 10.5, was the last Mac OS X release to support PowerPC processors.

Leopard launched on October 26, 2007, as a universal release capable of running on both Intel-based Macs and supported PowerPC Macs. Its successor, Mac OS X Snow Leopard 10.6, dropped native PowerPC Mac support and ran only on Intel hardware. That transition makes Leopard the final operating-system version for PowerPC-based Macintosh computers.

Leopard still required newer PowerPC hardware than earlier releases: it dropped support for G3 processors and required a G4 running at least 867 MHz or a G5. It also introduced features such as Time Machine, Spaces, a redesigned Finder, and the Stacks feature in the Dock.

Do not confuse hardware support with software compatibility. Snow Leopard could run on Intel Macs and use Rosetta to run many PowerPC applications. However, it could not boot on PowerPC Macs themselves, and OS X Lion later removed Rosetta.