Mini vMac emulates a Macintosh Plus, one of the earliest of the Macintosh computers. It can run old Macintosh software that otherwise couldn't be used on recent machines. Mini vMac requires a ROM image file to run, and so can be legally used only by those who own a Macintosh Plus.
VICE is a Versatile Commodore Emulator, i.e. a program that runs on a Unix, Win32, MS-DOS, Mac OS X, Amiga, or RiscOS machine and executes programs intended for the old 8-bit Commodore computers. The current version emulates the C64, C128, VIC20, Plus4, C64-DTV, all the PETmodels, and the CBM-II (aka C610).
ADTPro transfers disks to and from Apple II and Apple /// computers and the modern world using any of these communications methods: serial/USB, UDP via the Uthernet or LANceGS Ethernet cards, or audio via the Apple's cassette ports. ADTPro has comprehensive bootstrapping support for otherwise diskless Apple IIs. The home page includes extensive tutorials for getting started.