Go to "Players", and then "Cheats". Enter one of the following codes...

Missile launch detected A-Bomb Launcher
Call in artillery Airstrike
up up down down left right left right b a select start All weapons
Ive got jungle fever, shes got jungle fever Amazon Jungle
I dont have time to bleed Anytime
Beta Auto laser rifle
Wash away the rain Black Hole Generator
Australians all let us rejoice Bladerang
John Rambo Bow and arrow
Aint no mountain high enough Canyon Lands
Old Faithful Chain gun
This is my boomstick Double shotgun
Moonshine Drunken launcher
Shrapnel Flak Cannon
Gloop Goo gun
Follow the leader Guided launcher
Alpha Laser rifle
Gamma Laser shotgun
You are the moon master Level select
iopred Programmer message
DayDream Programmer message
Its a Jackal Sniper rifle
Bass in your face Soundwaave
9 Volt Spark Plug
Who rides the wrecking ball Zones 4 and 5

Chain gun - To get the Chain gun early, go to the training level and follow the instructions until you get to the ladder. Look to your left. You should see a wall. If you shoot the wall it will break. Keep shooting until it will not break anymore. The Chain gun will be there. If you have already unlocked the Chain gun, you will just find an ammo drop.

Extra weapons - In levels 2-1, 2-2, and 2-3, you can get a machine gun and/or machine gun ammo clip from every heli you destroy. Get the heli down to enough health, so that it will only require one grenade to kill it. Use Hyper Jump (hold [Modify] and press [Jump]) to get very close to the heli. You must be close enough to touch it. Use a grenade to finish it off. The heli will drop a machine gun clip with only 25 ammo in it. Note: This will not effect any of the weapons that the helis usually drop; every other time you use this trick you will get two items.

FlakCannon - In order to unlock the FlakCannon you must go to the training area. Then go to where you can shoot the targets. Go into top of that "tower" and use Time Distort (everything slows down). Do a Hyper Jump to the platform where that gun is located.

Googun - In level 2-2, walk right on the screen after the first metal bridge. You will go up an incline. Once at the top, there is a stump. Jump it and there will be another. Hold [Modify] and crouch to fall, then shoot to the right to unlock the Googun.

Machine gun ammunition - Finish off any enemy during a hyperjump with any weapon. You will automatically receive machine gun ammunition.

Spark Plug gun - In level 1-3 there is a bridge. Hold [Modify] (default is [Space]) and [Down] on the right side of the bridge. You will drop down. Then, use any weapon on the right side of where you dropped down to break the five sections of the wall to unlock the Spark Plug gun.

Soundwave gun - On level 3-3, go to the second metal bridge and use [Modify] (button can be found in pause menu, which is P). Do it again and you will land on a third bridge very low down. Shoot to the right as far as you can go, then shoot up. You will find the Soundwave gun.