One of my friends recently discovered Alt-Tab on Windows, which has been available for a very long time. However the utility of the default Alt-Tab behavior is quite minimal. There are two good alternatives that greatly enhance the default Alt-Tab behavior.
The first is the Alt-Tab Replacement (TaskSwitch.exe) from Microsoft which is part of the PowerToys for Windows XP bundle. This improves the Alt-Tab experience to include previews of the windows as one cycles through them. Useful, but not much of a leap compared to the default behavior. Keep in mind that the PowerToys are not supported by Microsoft.
The second alternative is TaskSwitchXP from ntwind software. TaskSwitchXP shows a list of windows with previews with the ability to take actions on them using both the mouse and keyboard shortcuts. It also has a rich set of keyboard shortcuts. Definitely a far better alternative to the default Alt-Tab and the PowerToy.