Well, this is a fun chart:
Currently, Apple only ships Intel-based machines. Mac OS X for Intel was released in 2006. The Intel version had been “leading a secret double life” since 2000, i.e. Mac OS X existed for Intel all the time, but was not released. In that time, Mac OS X was never self-hosting; instead, it was cross-compiled on PowerPC Macs. The first released version of Intel Mac OS X was a version of 10.4 for the Pentium 4 based “Developer Transition Kit” in 2005.
The first version of what would later be Mac OS X was “Rhapsody DR1″ released in 1997. It ran on PowerPC 604 Macintoshes (the 603 was not supported because it lacked a hardware pagetable walker) and was cross-compiled from OpenStep 4.2 running on Intel Pentium II CPUs. Rhapsody, which was basically OpenStep 5.0, also continued to run on Intel, but as mentioned before, Intel became a second-class architecture. Actually, somewhere between Rhapsody and OS X 10.0, there was a time when the GUI was not built for Intel.