I tested every trick/secret before posting it here. And I really enjoyed it. I hope this will end up my addiction with Window Games. It really works.

FreeCell

Secret - Instant Win
Instructions - Hold down Ctrl + Shift + F10 during game play. Then you will be asked if you want to Abort, Retry or Ignore. Choose Abort, then move any card to instantly win.

Secret - Hidden Game Modes
Instructions - In the "Game" menu choose "Select Game". Enter -1 or -2 to activate the hidden game modes.


Hearts

Secret - Show All Cards
Instructions - Edit this registry key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Applets\Hearts and create a new String value named ZB with a Data value of 42 . Start Hearts and Press Ctrl + Alt + Shift + F12 to show all the cards.

Background - This secret is a reference to Douglas Adams' book the Hitch Hiker's Guide to the Galaxy. 'ZB' is the initials of the character Zaphod Beeblebrox, the Galactic President. '42' is the answer to The Ultimate Question Of Life, the Universe and Everything.


Minesweeper

Secret - Reveal Mines
Instructions - Minimize or close all running applications. Launch Minesweeper, then type xyzzy. Next hold down either shift key for one second. Now when you move the mouse cursor over a Minesweeper square you will see a tiny white pixel in the top left corner of your desktop screen. This pixel will change to black when your mouse moves over a mine. You may need to change you desktop background to a solid color other then white or black to see the pixel.

Secret - Stop Timer
Instructions - Launch Minesweeper and start a game so the timer starts counting, then press the Windows Key + D to show the desktop. Now when you select minesweeper from the taskbar you can continue playing with the timer stopped.


Pinball

Secret - Extra Balls
Instructions - Type 1max at the start of a new ball to get extra balls.

Secret - Gravity Well
Instructions - Type gmax at the start of a new game to activate the Gravity Well.

Secret - Instant Promotion
Instructions - Type rmax at the start of a new game to go up in ranks.

Secret - Skill Shot
Instructions - Launch the ball partially up the chute past the third yellow light bar so it falls back down to get 75,000 points. There are six yellow light bars that are worth a varying amount of points:

First: 15,000 points
Second: 30,000 points
Third: 75,000 points
Fourth: 30,000 points
Fifth: 15,000 points
Sixth: 7,500 points

Secret - Test Mode
Instructions - Type hidden test at the start of a new ball to activate Test Mode. No notification will be given that this is activated but you can now left-click the mouse button and drag the ball around. While in test mode press the following keys for more secrets:

H - Get a 1,000,000,000 High Score
M - Shows the amount of system memory
R - Increases your rank in game
Y - Shows the Frames/sec rate

Secret - Unlimited Balls
Instructions - Type bmax at the start of a new ball. No notification will be given that this is activated but when a ball is lost a new ball will appear from the yellow wormhole indefinitely. Once this is activated you will be unable to activate other secrets without restarting the game.


Solitaire

Secret - Instant Win
Instructions - Press Alt + Shift + 2 during game play to instantly win.

Secret - Draw single cards in a Draw Three game
Instructions - Hold down CTRL + ALT + SHIFT while drawing a new card. Instead of drawing three cards you will only draw one.

Source: http://mywebpages.comcast.net/SupportCD/XPSecrets.html