When you have several programs open, they tend to overlap and hide information you need to see. You can close programs (and Modern UI/Metro-style apps in Windows 8) using keyboard shortcut Alt+F4.
In all versions of Windows, you can use keyboard shortcut ALT+TAB to scroll forwards through open windows/applications and Desktop or use Alt+SHIFT+TAB to cycle backwards. Release keys to bring currently selected window to front.
In Windows Vista, 7 and 8, you can cycle through open windows only (excluding the minimized ones) with keyboard shortcut ALT+ESC.
Windows Vista and 7 also include Flip 3D that shows previews of windows and Desktop in three-dimensional style. In Windows 8, the following shortcut keys cycle through open Modern UI apps and the Desktop only. Use keyboard shortcut WINDOWS KEY+TAB to scroll forwards through open windows or Windows Key+SHIFT+TAB to cycle to backwards. Release keys to bring currently selected window to front.
If you don't want to hold down WINDOWS KEY while cycling through windows, use keyboard shortcut CTRL+WINDOWS KEY+TAB to activate Persistent Flip 3D. Now you can use both arrow keys or TAB and SHIFT+TAB combination to cycle through windows. Press ENTER key to bring the currently selected window to front.
Windows 8 users can also cycle open Modern UI apps and Desktop by moving mouse pointer to the top left of screen and clicking the small preview windows that appears. Moving pointer downwards to the small rectangle on the left edge opens full list of running Modern UI apps.
Touch screen users can swipe in from the left edge of screen to cycle between open Modern UI apps and the Desktop. This is called App switching. To reveal a full list of open apps, swipe in from the left edge and drag back to the left a little without lifting the finger.
Windows Vista users have easy access to Flip3D on Quick Launch Bar - the Switch between windows button.
And this is how Flip3D and cycling through Metro apps look like:
This all is good for displaying only one window at a time, but sometimes it is necessary to see two or three or four or even more windows, all at the same time. The effectiveness of this depends, of course, on your screen/display resolution - the higher the screen resolution (for example, 1440 x 900 is higher than 1024 x 768), the more information you can see. Screen resolution is always width in pixels x height in pixels.
In Windows 8, most of the following applies to Desktop applications only. You can still display two Metro-style apps side by side, but only if display resolution is set to at least 1366 x 768. This is covered in the end of current article.
Remember, to simulate mouse right-click on touch screens, touch and hold.
Cascading open windows
You can always try cascading windows first. Cascading aligns all open windows so that you can always return to any open window by clicking on its Title bar or any corner you can see.
To cascade windows, right-click on an empty area of Taskbar and click Cascade Windows on the menu:
This command aligns them like in picture below. As you can see, you can always click on any of Title bars to activate a window and bring it to front.
If you don't like cascade, you can right-click on an empty area of Taskbar again and click on Undo Cascade command.
Showing windows side by side / Tiling windows horizontally
To tile open windows horizontally, right-click on an empty Taskbar area and choose Tile Windows Horizontally (in Windows XP) or Show Windows Side by Side (in Windows Vista, 7 and 8).
In case you need to undo the operation, right-click on an empty Taskbar area again and choose Undo Tile or Undo Show Side by Side.
Here are windows tiled horizontally, or side by side.
Showing windows stacked / Tiling windows vertically
To tile windows vertically, right-click on an empty Taskbar area and click Tile Windows Vertically (in Windows XP) or Show Windows Stacked (in Windows Vista, 7 and 8).
In picture below three windows are stacked, or tiled vertically. You can undo the operation by right-clicking Taskbar and choosing Undo Tile or Undo Show stacked.
When you have more than two windows open, the effect of tiling depends on your screen resolution. I have 1024 x 768 (width in pixels x height in pixels) resolution and such small resolution makes most of windows' contents unusable.
If you have more than three windows open, it does not really matter whether you tile windows horizontally or vertically - the result will look the same.
Minimizing all open windows
When you need to minimize all open program windows, you can click the Show desktop button on the Quick Launch Bar (in Windows XP and Windows Vista). In Windows 7 and 8, the button is on the very right of Taskbar's Notification area (aka System Tray).
You can also right-click on an empty Taskbar area and choose Show the Desktop. You can also use keyboard shortcut WINDOWS KEY+D for this. Keyboard shortcut Windows Key+M minimizes all open windows and Windows Key+Shift+M restores these.
Moving and sizing open windows in Windows 7 and 8
Windows 7 added several very useful keyboard shortcuts that are also available in Windows 8. If you need to minimize all other windows except for the one you're using, press WINDOWS KEY+HOME or click and hold left button of your mouse on a window's Title Bar and just shake the window. Cool, huh? 
If you want to see two windows side by side, you can also select a window and press WINDOWS KEY+Left ARROW KEY to move the active window to the left part of screen. Using the shortcut keys again restores the window's previous size and position. You can also drag the window to the left side of screen with mouse.
Naturally, press WINDOWS KEY+Right ARROW KEY for an active window to move it to the right part of screen. Or use your mouse to drag the window to the right side of screen. 
Pressing WINDOWS KEY+UP ARROW KEY maximizes current window. You can achieve the same by dragging the window to the very top of the screen with mouse.
To restore window's previous size, press WINDOWS KEY+DOWN ARROW KEY or drag the window down a bit with mouse. Pressing WINDOWS KEY+DOWN ARROW KEY again minimizes the window.
If you have multiple monitors, you can move windows between them. Use WINDOWS KEY+SHIFT+LEFT ARROW KEY to move a window to the left monitor and WINDOWS KEY+SHIFT+RIGHT ARROW KEY to move a window to the right monitor.
Organizing Modern UI-style apps in Windows 8
In Windows 8, above instructions work only if you're using Desktop applications. All new Modern UI/Metro-style apps can be displayed only side by side, but just two at a time. One of these apps always takes about two thirds of screen estate.
You also need to set screen resolution to at least 1366 x 768 for the following to work. If display resolution is lower, only one Windows 8-style app can be shown at a time.
First, use keyboard shortcut Window Key+Tab to open a list of open Metro apps. Pressing Ctrl+Windows Key+Tab instead makes the menu stick to make selecting somewhat easier.
Touch screen owners should swipe in from the left edge of screen and immediately drag back a little to the left to reveal the list.
Now drag an item to the left or right corner of the screen.
The app will then get about one third of screen space, called gutter. To resize it to two thirds, use the slider between the two open apps. As you can see, even all running Desktop programs change their size to fit in the tight space.
To set one of the running apps full-screen again, drag the slider all the way to the left or to the right. You can also use keyboard shortcut Windows Key+. to move the smaller app to the right - first time makes the app full-screen, second time moves it to the right corner and gives it about 2/3 of screen space, third time moves it back to the left corner with 1/3 of screen space. Keyboard shortcut Windows Key+Shift+. does the same, but cycles to the left.
