SheepShaver goes back to when BeOS was on PPC. It allowed for PPC virtualization and later moved onto emulation. SheepShaver emulates a PPC CPU on an x86 machine, and provides virtualization on a PPC machine (nice for some apps and games that never would run right in classic). It's an open source project now and runs on Linux, OS X, and Windows. It does not run OS X. It's fairly straight-forward to get going; the only glitch is getting the right ROM / OS combo.
I have a couple apps I never upgraded to OS X, and don't use frequently enough to justify the price to purchase. Plus I want to have 25 years worth of Mac software still at my fingertips no matter where I'm at.