Although XP has a Virtual Desktop PowerToy, there's no version for Windows Vista yet. However, there are not one, but three third party programs that Cnet just reviewed: VirtuaWin, which supports up to 20 separate desktops, Virtual Dimension, and Z-Systems' Vista/XP Virtual Desktop Manager, which takes advantage of Vista's Aero interface.
Out of these, the Z-Systems program looks like the one I would choose, not only because of the Aero aspect, but also because it offers full-screen desktop previews in Vista so that you can easily find the desktop you're looking for. With Z-Systems, you can generate a system tray icon for each desktop, which makes switching really fast. You can also select certain programs that will show up on all of your desktops.