Extra weapons: 

Type baabaa at the weapons option screen. Exploding sheep, the banana bomb, and full motion with the ninja rope will be available during game play. Type aabaab at the weapons option screen to disable cheat mode. 

 

Unlimited Sheep: 

Type bobjob at the weapons options screen. 

 

Rapid fire missiles: 

Type aabaab while shooting a mini-gun. 

 

Extra turn: 

Type aabaab while firing a banana bomb. 

 

Unlimited bazooka missiles: 

Select a target with the Uzi, then hold [Shift] + [F1] and press [Space].
The funniest levels are under these codes:

70345 
223981979 
7373888390 
4802043 
And then a couple of level codes:

Level 02 -- OHSOEASY 
Level 03 -- UNCHAINS 
Level 04 -- BJORNPOP 
Level 05 -- DODGEMAX 
Level 06 -- STARTURN 
Level 07 -- HEYGEOFF 
Level 08 -- REZDOGGY 
Level 09 -- HIGHKICK 
Level 10 -- LONG AGO 
Level 11 -- HEY JOHN 
Level 12 -- SATANICS 
Level 13 -- NEARDEAD 
Level 14 -- SCUMBAGZ 
Level 15 -- GOOD BYE 
Level 16 -- TRUTHOUT 
Level 17 -- 17THKIND 
Level 18 -- GOGOGOGO 
Level 19 -- PIGSPACE 
Level 20 -- TRUMPTON 
Level 21 -- PARANOID 
Level 22 -- MIXTURES 
Level 23 -- VERY MAD 
Level 24 -- INSANITY 
Level 25 -- GREATGIG