macOS 8, not to be confused with OS X 10.8—which you’re probably much more familiar with—is a 1990s-era operating system that found a home on ancient Apple systems with decidedly old-school names, such as “Power Macintosh” and “PowerBook.” But now, you can use it for free on any modern Mac or Windows PC—or at least until Apple gets angry and goes after the project that makes it possible.