Windows: There's no shortage of applications that add virtual desktops to Windows, but nSpaces is one of the easiest to use and easiest to set up, complete with hotkeys for desktop switching, program memory so apps always open on the right desktop, and even the ability to password protect virtual desktops for security.
What makes nSpaces stand out from other apps that add virtual desktops is that it fits right in with the Windows Vista and Windows 7 style. nSpaces can assign colour profiles and different wallpapers to each virtual desktop, and can give you colour cues when you move from the desktop with, for example, all of your entertainment apps on it to the one with all of your work apps on it.
nSpaces lets you add additional virtual desktops in columns and rows, tweak the key commands that move you among your virtual desktops, and give your desktops names and labels. This way you'll never forget which desktop is used for which tasks. The utility also remembers which applications you assign to each desktop, so they always open in the right place. You can also password-protect virtual desktops for a little added security.
nSpaces is free, and available for Windows XP, Vista and 7. What application do you use for virtual desktops? Share your tips in the comments.