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I've always loved being able to run multiple operating systems at once. I run Windows XP as my main OS, and usually a Linux distro in either VMWare or MS Virtual PC. Now that Macs are Intel based I may even try emulating OSX. If you're not familiar with how to do this, I would recommend taking a Virtualization Course before just jumping into it blindly. There can be huge productivity gains, but you really should know what you're doing.