SheepShaver is an open source PowerPC Apple Macintosh emulator. Using SheepShaver (along with the appropriate ROM image) it is possible to emulate a PowerPC Macintosh computer capable of running Mac OS 7.5.2 through 9.0.4. Builds of SheepShaver are available for Mac OS X, Windows and Linux.
SheepShaver is considered a good replacement for the Classic Environment which is not available in the most recent versions of Mac OS X.
SheepShaver began life in 1998 as a MacOS emulator for BeOS. At that time, SheepShaver was a commercial product developed by Christian Bauer. In 2002, following the commercial decline of Be, SheepShaver was released as an open-source application. Development at that time was driven by Gwenole Beauchesne, who ported the emulator to Windows, Linux and Mac OS X.
Beauchesne suspended his work on SheepShaver in April of 2008, although volunteers have contributed bug fixes and features since that time. Builds based on those contributions are listed on this site and in the SheepShaver forum.
The name of the emulator is a reference to ShapeShifter, a Mac II emulator for AmigaOS.