Type in the word:
HUIBON - All weapons
VONLUX - More life
OTOREL - Auto Reload
VENRUF - Stops Virus
TIXOBO - Skip Stage
For PSX and PC: Enter COMMANDO (as a password) - You will be transported to a ring where you fight yourself to the death.

Enter ACTIVATE on the password screen and start with 99 chips and 4 batteries.

Other Tips:
In the roman-empire try searching the baskets in level-1.

World War 2: When the man with the machine-gun shoots you from above try searching the trash can, then fire a bazooka missle into the window of the man who shoots...

Future - Level 2: After you defeat the main boss, don't pull the lever to end the stage before putting on the space-uniform or else..

Modern wars - Level 2. When you first start out there will be a dog sitting in front of an old car, DONT SHOOT IT!!! Go up to it and click on it, it will get up and show you all the secrets of the level. Just follow it! It can be killed so protect it.
 
 

Thanks to Revolution readers Kauza, Michael Lee, Nadav, Joe Gibson, The Gatekeeper , and Hisham Adamjee!!