Virtual PC started off originally as an x86 emulator for PowerPC Macintosh to run MS-DOS and Microsoft Windows. Connectix, the company that made it, was purchased by Microsoft. Virtual PC was then retooled into a virtualization tool for x86 systems. Microsoft discontinued Virtual PC in favor of a server-oriented virtualization product called Hyper-V.
SheepShaver is a MacOS run-time environment for BeOS and Linux that allows you to run classic MacOS applications inside the BeOS/Linux multitasking environment. This means that both BeOS/Linux and MacOS applications can run at the same time (usually in a window on the BeOS/Linux desktop) and data can be exchanged between them. If you are using a PowerPC-based system, applications will run at native speed (i.e. with no emulation involved). There is also a built-in PowerPC emulator for non-PowerPC systems.
ScummVM is a program which allows you to run certain classic graphical adventure and role-playing games, provided you already have their data files. The clever part about this: ScummVM just replaces the executables shipped with the games, allowing you to play them on systems for which they were never designed! ScummVM is a complete rewrite of these games' executables and is not an emulator. 153554b96e