Area 51 Cheats

Become a rebel Kronn warrior:
Gund down the first three S.T.A.A.R. team members without gunning down anything else. When the first human is shot, you will not be harmed by any gun fire. After the third human is shot, you will transform into an alien in the main hallway of the first level.  Infravision accompanies this alien  which allows targets to be more visible.

Baby Aliens:
Upon entering the building from the helicopter, gun down the exit sign when you see the purple alien, now gun down the first three lights. You will now have the chance to gun down baby aliens before they take adult form.
User Cheat by BruceBruce  

Get to the Tank Top room:)
Gun down the first fifteen windows on the right side of the hanger in wave two, right after you enter. Two players are needed for this to be accomplished. You will also need to use multiple views as well. If done correctly the game will automatically lead you to a room called Tank Top.

Get to the X Marks The Spot room:
Gun down the first ten hanger windows while outside on the tarmac. The first two windows must be shot quickly,  right after the game starts. If you do this correctly the game will lead you to a room called X Mark The Spot.
User Cheat by BlowOut  

Get the shotgun:)
In the warp ahead level, get  to the part when it says SELF DESTRUCT. You will turn, a blue blinking light will appear. Shoot this particular blinking blue light and you will receive the shotgun.

With the shotgun you do not have to aim exactly on target and you will still annihilate the enemy.

Go to the secret rooms:)
Activate the SECRETS ROOMS option. Do one of the actions below to go into a room called Chow Palace. The Chow Palace is a locations filled with various power-ups.

Inside the bunker, shoot all the blue lights on the left wall.
Shoot out all the back windows inside the bunker.
When riding inside the forklift, shoot all the yellow barrels.
When riding inside the jeep, shoot all the yellow boxes.
When you are in the building shoot the name plate off the second door as it is opening.
When you are being bombarded by the two aliens on top of the hill, shoot everything yellow.