Microsoft Virtual PC -- it's free -- but the OS to install on the VM might not be (e.g. if you're going to run Windows on the VM you should have a license for it). Came in handy as it allows me to have an older OS (e.g. DOS, DOS w/ Win3x, Win95/98/Me) without having to maintain an older computer for which I can't find parts (not only that, but it saves on desk clutter as well). Virtual PC also comes in handy when you need a test environment, where reconfiguring a VM is much simpler than changing an actual physical computer.
There is one other bit too -- if you find yourself need to support Win95 and earlier OS, you might want to get the 2004 edition of Virtual PC and extract the disc image file for the mouse utility (the 2007 edition doesn't support Win95 and older). Although it's not absolutely necessary, but it gets tiresome to have to "capture" and "release" the mouse pointer from the VM (the add-in allows you to move the mouse in and out of the VM w/o having to deal with manual "capture" and "release"). Some of you might wonder why I'd even want to use Win95. We had a customer who was using it and was having difficulty with our software in that environment. Sure, it's easy to say "you should upgrade," but payroll departments (the customers we deal with) often get "hand-me-down" computers (general logic being that payroll departments are cost centers that don't generate income, so they're often given minimal resources). It's one of the reason why we still had to support 5.25" floppy for a long time.

As an aside... does anyone know if foxprowin 2.6 apps run w/ wine under Linux?