When you complete the indicated number of 'Emergency' Situations the corresponding bonus weapon will become unlocked.

Unlock Shotgun Mk. 1:
Complete 1 Emergency situation.

Unlock Assault Rifle Mk. 1:
Complete 2 Emergency situations.

Unlock Shotgun Mk. 2:
Complete 3 Emergency situations.

Unlock Assault Rifle Mk. 2:
Complete 4 emergency situations.

Unlock Shotgun Mk. 3:
Complete 5 emergency situations.

Unlock Assault Rifle Mk. 3:
Complete 6 Emergency situations.

When you complete Webster Street the Thermal Breather becomes unlocked. The Thermal Breather prevents you from losing health from inhaling fumes and also picks out heat sources so you can find people easier.

When you complete ALL the Story missions in the game on any difficulty setting 'Terror' mode will become an available option. Terror mode has other levels of medals.

When you capture the indicated number of gang leaders alive the corresponding Emergency mission will become unlocked.

Unlock Emergency Downtown Mission:
Capture 1 gang leader alive.

Unlock Emergency South Side Lot Mission:
Capture 2 gang leaders alive.

Unlock Emergency Dockyard Mission:
Capture 4 gang leaders alive.

Unlock Emergency Sewers Mission:
Capture 6 gang leaders alive.

Unlock Emergency Thompson St. Mission:
Capture 8 gang leaders alive.

Unlock Emergency Angel Heights Mission:
Capture 10 gang leaders alive.

At the Main menu enter Up, Up, Down, Down, Circle, Down, Up, Circle then select the cheat section and enter the following codes to unlock the corresponding effects. These codes are case sensitive.

Unlock ALL Levels and Emergencies:
KEYTOTHECITY

Unlock Long Range Stungun:
FRYINGTIME

Unlock Mini gun (replaces the Pistol):
MINIFUN

Bullets Set Fire to Enemies (similar to one hit kills):
BURNINGBULLET

Thermal Breather MK2 (for visibilty in smoke):
ISEEYOU

Unlock MK3 Assault Rifle with Unlimited Grenades:
ULTIMATEPOWER

Unlock MK4 Pistol:
ZEROTOLERANCE

No Heads:
KEEPYOURHEAD

Activate Disco Mode:
DANCINGFEET

Activate Terror Mode:
BURNERSREVENGE

Helium Voices:
WHATWASTHAT

Unlock Extra Pistol Clip:
Collect 2 Medals

Unlock Shield Mk. 2:
Collect 4 Medals

Unlock Pistol Damage Upgrade:
Collect 6 Medals

Unlock T-Zero Body Armor:
Collect 8 Medals

Unlock Smoke Grenades:
Collect 10 Medals

Unlock Stun Gun Mk. 2:
Collect 12 Medals

Unlock Riot Grenades:
Collect 14 Medals

Unlock Second Extra Pistol Clip:
Collect 16 Medals

Unlock Shotgun Damage Upgrade:
Collect 20 Medals

Unlock Grenade Belt:
Collect 24 Medals

Unlock Extra Shotgun Clip:
Collect 28 Medals

Unlock Assault Rifle Damage Upgrade:
Collect 32 Medals

Unlock Pistol Mk. 2:
Collect 36 Medals

Unlock Extra Assault Rifle Clip:
Collect 40 Medals

Unlock T-Zero Battle Armor:
Collect 45 Medals