Hot on the heels of Apple's Boot Camp announcement, Parallels has announced Parallels Workstation 2.1 Beta for Mac OS X, a beta of their virtualization software for Intel Macs. Their software allows you to run Windows in OS X, much like Virtual PC, without the need to actually boot into Windows.
Using their software you can run lots of version of Windows (3.1, 3.11, 95, 98, Me, 2000, NT, XP, 2003), any Linux distribution, FreeBSD, Solaris, OS/2, eComStation, or MS-DOS and for the moment the software is free (and in beta). When it is out of beta the software is expected to cost $49.99
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