Type "SOYINCAPAZ" as a name to enable cheat mode. Then, press one of the following keys during game play to activate the corresponding cheat. Cheat mode can also be activated during a mission by typing "SOYINCAPAZ" after selecting a commando.

Code - Result:

[Ctrl] + I - God Mode
[Ctrl] + V - Invisibility
[Shif + X - Place Commandos Under Pointer
[Ctrl] + [-] - Show Frame Rate
[Ctrl] + [Shif + N - Skip Mission
[Ctrl] + [Shif + X - Destroy All Opponents
Hit CAPS then CTRL + SHIFT + X. - Kill nazis

NS2B7 - Mission 1
GMD1T - Mission 2
NMD15 - Mission 3
YDNCQ - Mission 4

Then you find yourself in the Berlin sewers after the cut scene in which you meet the Spy, you can easily overcome each room of the sewer and clear it out by simply using punches, knifing and running into a room, being spotted and then running out, and taking cover behind your fellow commandos, who are of course, waiting on guard move with their guns pulled out, aimed at the door. When the Nazis burst in, your men nail them, and you can clear all the sewers out doing this. With a couple of the Nazis, it's even easier to simply come out behind them, knife them and drag their body back through the door.

I completed this level first of all by literally taking out everyone a man at a time, and boy was it hard going! Then my brother tried it, and discovered that when you've sniped one nazi from the Russian embassy (be careful, pop out of the wrong window and the elite sniper will kill you!), and cleared out the Nazi riflemen around the fountain by sniping them from the unoccupied yard area (behind the wall, above the little dugouts the Russians soldiers mostly start in), you can crawl down, take their stuff and then as you move down towards the highest tower (it's a church building), the elite nazi sniper should move his position and make a run for it; he tends to respond to your movements to keep you covered so you can't snipe him but he can snipe you. As he runs, you can actually quite safely change direction, bolt the other way and snipe him as he's running - ending this part of the mission early!