You may be able to run older versions of IBM's AIX (especially the Apple Network Server version) on QEMU or PearPC.

And if you're really persistent and know exactly the right people, you might even be able to find copies of Windows NT 4, Novell Netware, and Mac OS Copland that might run, although don't count on it - IIRC WinNT/ppc wasn't popular and Copland never made it out of internal beta testing at Apple. Linux and most of the *BSDs support PowerPC systems, making them good choices for putting old Macs back in service as file servers or whatever (in fact, FreeBSD founder Jordan Hubbard is the head of Apple's Unix development operations). Mac OS Classic through version 8.1 is available for free from Apple (fair warning to Mac newcomers: classic MacOS is very different from OS X on almost all levels), and some old development tools like CodeWarrior, MPW, and MacApp are available on abandonware sites. Although by far the best known Power systems are the Power Macs and the video game consoles, Power is pretty well supported by operating systems. OpenEmu is an all in one emulator for the Mac and can many run game consoles including NES, SNES, N64, Game Boy Advance, Nintendo DS, Sega Genesis, and many more How-to Install N64 Emulator to Mac.

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