VirtualBox may be free, but you still have to own a copy of Windows. If you're not willing to shell out a few hundred dollars to Microsoft but still want to run Windows on your Intel-based Mac, there is one more alternative: CrossOver from CodeWeavers costs $40 and runs Windows applications on Mac OS X -- without Windows.
CrossOver is not virtualization, as evidenced by the whimsically named technology behind it: the open source Wine (Wine Is Not an Emulator) project. With CrossOver, Windows applications run directly in Mac OS X, and not in a virtual machine. Wine is an implementation of the Win32 API on Mac OS X. The Windows applications don't know they're not running on Windows. There's also a CrossOver for Linux.
The result is that a Windows application running with CrossOver uses fewer system resources, including memory, disk space, and CPU utilization, than the same app running in Parallels Desktop or VMware Fusion. Performance of Windows applications is very good.
[ Parallels Desktop 5, VMware Fusion 3, or VirtualBox 3.1? See "InfoWorld review: Windows on the Mac." ]
Parallels Desktop and VMware Fusion can hide the Windows desktop and enable you to move data between the Windows and Mac environments, but they still use Windows' NTFS file system in the form of a virtual C: drive. With CrossOver, Windows apps share the Mac's file system. CrossOver lets you launch not only Windows files, but also Windows applications, directly from the Mac OS X Finder. CrossOver doesn't hide Windows -- it jettisons the operating system.
There is a catch, however: Not all Windows applications run in CrossOver. In fact, quite a few don't. That's because CodeWeavers and the Wine developers tweak Wine for specific programs.