Q is a Mac OS X Cocoa port of the excellent open source CPU emulator QEMU. QEMU is a generic and open source processor emulator (or virtual machine) that allows you to run, e.g., Linux on a Windows PC or Windows on a Linux PC, etc.
It may not be as efficient as Parallels but it's free and achieves a good speed on Intel Macs, giving the performance of about a 500Mhz PC in x86-on-x86 emulation so you might just get away with running Windows XP on it. Like Parallels, and Virtual PC before it, you can quickly switch between guest systems and save and restart guest systems at any stage. You can also easily exchange files between host and guest systems.
While Q makes use of Mac OS X advanced technologies like OpenGL, Quartz, Coreimage and Coreaudio to provide maximum acceleration, it'll never replace a real PC but it could be useful for testing websites or utilities under Windows or Linux.
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