Hitman Contracts:

Unlock all Levels:
Enter the following code at the main menu: 
X, Y, B, Left, Up, Right, L, R.

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. 

Dual Gold Desert Eagles: 
Find second Key Card Reader Door, and open with Orthmeyer's Key Card, take Dual Desert Eagles on floor inside. 

Minigun: 
Find first Key Card Reader Door, and open with Orthmeyer's Key Card, Kill Patient, take Minigun.

Each weapon is unlocked by completing the corresponding mission and achieving the Silent Assassin rating. 

AK 74 Silenced Assault Rifle Mission 9 
CZ 2000 Dual Pistols Mission 1 
GK 17 Dual Pistols Mission 10 
M4 Carbine Silenced Assault Rifle Mission 6 
Magnum 500 Dual Pistols Mission 4 
Micro Uzi Dual Submachine Guns Mission 2 
Micro Uzi Silenced Dual Submachine Guns Mission 11 
MP5 Silenced Submachine Gun Mission 8 
PGM Silenced Sniper Rifle Mission 12 
Sawed-Off Shotgun Dual Mission 5 
SG220 .S Dual Pistols Mission 7 
Silverballer Silenced Dual Pistols Mission 3 

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.

Level Skip/ Silent Assassin Rating: 
Anytime in gameplay except training:
Press R-Trigger,L-Trigger, Up, Down, X, A ,L-Thumbstick click, B,A,B,A.