Hitman Contracts:

Unlock All Levels:
At the main menu press X, Triangle, Circle, Left, Up, Right, L2, R2.

Or:

Unlock All Levels Without Cheats: Start a New Game and when it asks you to choose a difficulty go back and select Current Game, and every level should be unlocked, without a rating.

Unlock Weapons:
Unlock each weapon by completing the corresponding mission and achieving the Silent Assassin rating.

Weapon   -   Level:
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.

Live Again:
When your health bar reaches zero the game will enter a black and white death sequence. If you are 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. 

Unlock the 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.