Select Multiple Windows on the Taskbar

Hold down the Ctrl key while clicking on a taskbar button, and then while still holding down the Ctrl key, click on another taskbar button. You'll notice that they are both selected. (Note that you can click on a button again to deselect it.)

Now that they are both selected, you can right-click on one of the buttons and choose to cascade, stack or show the windows side by side.

I don't find cascading or stacking very useful, but side-by-side is something I use quite often.

This can be extremely useful when you are trying to look at two windows at once, especially when you've got a ton of windows open but only want to tile two of them.

Note: This should work fine in Windows XP even though the screenshots are from Vista