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.