Press Enter while playing the game in single player mode

Code - Effect
fortknox - Additional $1,000
keyholemaster - Full map
yeepeekaye - Launch a super weapon (nuclear bomb) at the pointer
ineedalltechnos - Unlock entire tech tree and unlimited cash
motherrussia - Russian T-80 tank
coolihaveanewcar - CIA Armored SUV
swatatyourorders - S.W.A.T. member
greenjelly - British cop
blackhawkdown - SA12 Anti-Aircraft Missile Launcher
duckhunt - Duck
ymca - U.S. police officer
coolimthepresident - President of The United States of America
bringoutthedead - British Ambulance
bigbrother - Camera Mode