Cheat List
Open the hitmancontracts.ini file (it's in the folder where the Hitman game files are) and add the following lines to it:

ENABLECONSOLE 1
ENABLECHEATS 1

Then start the game and during gameplay press SHIFT + ESC. Change the cheat value from 0 (false) to 1 (true) to enable the cheat.
God 1 :Enables God Mode(God 0 deactivates it)
Giveall :Gives all weapons and items
Infammo :Gives Infinite Ammo for all guns
Nailgun 1 :Enables Nail gun (Nail gun 0 disables)
Invisible 1 :Makes you invisible to enemies*
Invisible 0 :Deactivates invisibility

Cheat: Cardboard Tube
Enter "IOIPENNY" after enabling cheats.

Cheat: Level Select
Open the HitmanContracts Configuation Settings file found in:

C:\Program Files\Eidos\Hitman Contracts

and change the following:
DefaultScene=AllLevels\Logos.gms
to
DefaultScene=AllLevels\levelmenu.gms

Save and play! You will get a level select menu.

Unlockable: Dual Gold Desert Eagles
After getting the key card from the Wang Fou Incident you most likely went to Asylum Aftermath to collect your Mini Gun. After that continue thru the level and when you get to a large room with windows and the door ahead... STOP! One patient will be walking around and on your right will be steps leading down to a door with red lights over it. Enter that room with the key card. Walk forward until you find the Gold Desert Eagle Guns right under your nose!

Unlockable: Mini Gun
The mini gun is held by a patient who is behind the locked doors of a basement room in Mission 1: Asylum Aftermath. The keycard that unlocks the doors can be found during Mission 9: The Wang Fou Incident. It is called Orthmeyer’s Keycard and is on Lee Hong’s desk in his office on the second floor of his mansion. After finding the card, finish the mission and save your progress. Quit to the main menu and replay the first mission. Head to the northern section of the map and turn right in the small room with the red flashing sensor on the ceiling. Pass through the west door and dispose of the patient in order to collect the mini gun.

Unlockable: Weapons
Each weapon is unlocked by completing the mission and achieving the Silent Assassin rating.
Mission 1: CZ 2000 Dual Pistols
Mission 2: Micro Uzi Dual Submachine Guns
Mission 3: Silverballer Silenced Dual Pistols
Mission 4: Magnum 500 Dual Pistols
Mission 5: Sawed-Off Shotgun Dual
Mission 6: M4 Carbine Silenced Assault Rifle
Mission 7: SG220 .S Dual Pistols
Mission 8: MP5 Silenced Submachine Gun
Mission 9: AK 74 Silenced Assault Rifle
Mission 10: GK 17 Dual Pistols
Mission 11: Micro Uzi Silenced Dual Submachine Guns
Mission 12: PGM Silenced Sniper Rifle

Hint: Come back to Life
When your health bar reaches zero the game will enter a black and white death sequence. If you are able to get four consecutive head shots on four different enemies during this sequence you will receive a small amount of health and return to the game.

Hint: Getting the M60 Early
On the level "Deadly Cargo," it is possible to get the M60. If you don't get it here, you will have to wait until the 2nd to last level. Make your way to where the SWAT sniper is, on the building overlooking the ship. After the SWAT team attacks the ship, a helicopter will begin circling the area. If you shoot the gunner on the ship, he will fall to the ground with his M60. Make sure that you assassinate the target first and you kill the gunner while the helicopter is over a land area that you can reach.

Hint: Master Key
In the level “Traditions of the Trade” when you are on the 3rd floor you will eventually see a vacuum boy. When he opens a door to vacuum the room, go to the door and pick up the master key. You should then have access to most doors.

Hint: Reflection Ghost
When playing The Meat King's Party look at the loading screen of the slaughter house. You should see a faint skull. This skull is also on the pinball machines in Rotterdam Rendezvous.

Hint: See Lee Hong
In the level Asylum Aftermath, once you get the double Desert Eagles, keep going forward through the second set of doors until you get to the last tank with a body in it, then look to the right and there is a tank with Lee Hong written on it.