Cheat mode:
Complete one of the following to unlock the corresponding feature, enable at the weapons or game options screens.

Worms bleed when shot....Earn gold medal in Basic Training mode
Aqua sheep....Earn gold medal in Super Sheep Racing training mode
Longbow power-up....Earn gold medal in Euthanasia training mode
Shotgun power-up....Earn gold medal in Rifle Range training mode
Grenade power-up....Earn gold medal in Artillery Range training mode
weapon crates have sheep....Earn gold medal in Crazy Crates training mode
Invincibility....Earn an Elite rank in Death match mode
Laser sight....Complete mission 4
Jetpack.... Complete mission 8
Fast walk.... Complete mission 13
Invisibility in multi-player....Complete mission 16 
Low gravity....Complete mission 20 
Indestructible landscape....Complete mission 25
Super Banana Bomb power-up....Complete mission 33
Full Worm game options....Gold medal and Elite rank in all.

Home runs:
Find an enemy worm standing close to the edge of the level. Use the baseball bat to hit him into the water to get a "home run" complete with baseball bat cracking sound, organ music and message "It's Outta Here" and "Bases Loaded".