Windows Vista has that feature build in that lets you flip through all the open Windows by pressing Alt + Tab. This feature is called Windows Flip in Windows Vista and is available in all editions of Vista except Home Basic.
Win Flip is a software for Windows XP that rebuilds this function so that you can make use of it under Windows XP as well. . Instead of accessing the feature by pressing Alt + Tab (update: it is also Windows + Tab in Vista) users press Windows + Tab instead. The only difference from what I can tell is that videos that are currently playing stop for as long as you are using the hotkey to browse through all the open windows and programs on your computer system.
Win Flip uses a surprisingly low amount of memory. Another nice feature of Win Flip are mouse gestures which provide you with an option to access the flipping mode the program makes available by drawing a circle with your mouse on the screen. You can then browse the open windows with the mouse wheel instead which is an interesting option if you prefer to keep your hand on the mouse at all times. A click with the mouse activates the window that is shown at that point in time so that it is moved to the foreground.
Update: Win Flip is no longer available on the project homepage, in fact, the homepage too is not available anymore. We have uploaded the latest release version of Win Flp to our server from where you can download the program. Just click on the following link to do so: Can your computer run the game?
We have tested the program under newer versions of Windows and found it to be working just as good as under XP. Note that you can right-click the program's system tray icon after launch to change some of the functionality it provides. Here you can for instance modify the keyboard shortcut, select slow flip which lets you move your hands off the hotkey without the display going away, or disable the mouse trigger or sensitivity.