Splinter Cell - Pandora Tomorrow:

LAX Room keycode:
If you want to get into the security room after the car park on the LAX level instead of using the vents in the toilet or distracting the guard to get him out and then finding out you have locked yourself in the room, just type 1387 on the keypad and your in!

Defeating Norman Soth:
After you have climbed up the elevator cables and have gotten on top of the other elevator, crawl through the hole in the wall. Norman Soth and two other mercenaries are on the top pathways. While in the hole, there will be one guard on the top path, slightly to the right. You should be able to shoot him in the head. Then, a timer will start and you will have one minute to kill Norman Soth and his other mercenary. When you shoot the first mercenary, drop out of the hole onto the first pathway. The second mercenary will go to exactly where the you shot the first mercenary. Shoot the second mercenary in the head. Note: The second guard will be aiming at the hole, so he will not see you if you drop out of the hole. The last person to kill is Normon Soth. Start running down the pathway and turn left on the second to last walkway on the left. Look up and turn on your thermal vision. You will see Normon Soth. Get a clear shot and shoot him in the head. After that you must retrieve the smallpox box from the top pathway. If you are on the second to last pathway on the left, go up the ladder. There will be another ladder; use it to climb up. Drop off the ledge onto the pathway. Run down the middle pathway to find the smallpox box. 

Better Aim:
With your pistol out, press Up to turn on the laser sight. The laser sight makes it easier to get headshots. Be careful though, if you are in dark corners and have the laser sight on, enemies will notice and attack you. 

Sticky Cam Knock Out:
If you're out of options and only have Sticky Cams left, you can shoot a Sticky Cam directly at an enemies head to knock him out. 

Better Stealth:
Crouching and walking slowly will keep your steps soft and make it very hard for enemies to hear or detect you. 

Make It Dark:
Shoot out lights with your gun to make yourself harder to see. Be sure not to shoot a light when an enemy is beneath it or they'll hear the shot. 

Whistling:
Press Black to whistle. Whistling will attract a guard, giving you an opportunity to attack them away from other guards or lights. 

Prince of Persia Reference:
On the train mission, knock out the guard in the second car, then go behind the fenced door. Look down you will see a magazine featuring Prince of Persia.

When you don't have your rifle yet and you can only use your gun (like in the first mission) click the right thumbstick to use your binoculars, use the crosshair to aim at the item you plan to shoot, click again to go back to normal view (do it gently so not to change your position) now whip out your gun and shoot, if done correctly you can hit lights from across the room :D

Second one is very handy, when hiding in a room for an alerted guard use your optical camera to view the other room, now when the guard comes to check out the room, he won't be able to open the door because you're using the camera, instead of freaking out and sounding the alarm he'll try a few times and walk away with a "it must be nothing".