NetBSD

NetBSD is an open source Unix derivative based on the Berkeley Software Distribution that aims for platform independence through clean separation between the hardware and the the kernel. It has been ported to many platforms from embedded systems to 64-bit computers.

NetBSD for classic Macs
The 68k port of NetBSD, originally known as MacBSD, was the first free operating system to run on a Mac. It requires a Motorola 68040, or a 68030, or a 68020 with a 68851 memory manager. Systems with a Motorola 68LC040 processor to not have full FPU support.

The PowerPC port of NetBSD supports most Power Macintosh or PowerPC-based Macs since 1995. It requires Open Firmware support.