Most Linux users are very familiar with virtual desktops (a.k.a. workspaces), which enables you to organize your opened applications in more than one desktop that you can switch over from one to another using hotkeys or mouse clicks.
Though virtual desktops feature is not bundled in MS Windows OS, its availability in the form of 3rd party software, either free or commercial, has been around for many years. Microsoft also provides a virtual desktops Powertoy called Virtual Desktop Manager (MSVDM), which many find it is slow, lack of functionality, and not performing as good as the 3rd party counterparts.
If you are looking for an open source virtual desktops for Windows, Virtual Dimension used to be a popular and good one, but its development seems to have ceased around 2005.
The virtual desktop I'm using now is VirtuaWin. It is free and licensed under GPL version 2, and latest release is Version 4.1 dated 09-09-2009.
VirtuaWin has been around since 1999. It can run in all version of modern Windows OS including Win 95, Win 98, Win ME, Win NT, Win 2000, Win XP, Win 2003, Win Vista and Win 7.
It is very lightweight, fast in desktop switching, modular with features expandable by loading more modules into it. It even has a portable version which doesn't need any prior installation to use.
VirtuaWin is very easy to install and run. By default, you will get 4 virtual desktops (2 x 2) which you can navigate by pressing Ctrl-Alt together with one of the arrow keys on your keyboard. If you don't like this hotkey setting, you can always change your preference in its setup.
You can increase or reduce the number of virtual desktops by configuring its Desktop Layout, up to a maximum of 20. The more virtual desktops you use, the more amount of memory will be consumed.
With VirtuaWin, now you can use, for example, one desktop for web browsing, one desktop for graphics editing, one desktop for document writing, one desktop for network monitoring, ... this is of course more organized than cluttering all of them in a single desktop in Windows.
If you are a developer, and find any feature lacking in VirtuaWin, you can in fact make the feature by yourself using the VirtuaWin module SDK. Being open source and modular, the feature list of VirtuaWin is believed to be able to expand unlimitedly with the support and contribution from the developers community.
In fact, some of the fancy features in other virtual desktops software has been made available to VirtuaWin via module contributions.