Activate Cheat Mode:
While playing press A + B + Start

Cheats to use in Cheat Mode:
Enter Cheat Mode (hold down A + B + Start) then do these cheats in game, without pausing. Preferably, you should have your fists equipped, so you don't accidentally blow yourself up or anything.

LEFT, RIGHT, UP, DOWN, A, A - All Weapons
LEFT, RIGHT, UP, DOWN, A, L - Armor
LEFT, RIGHT, UP, DOWN, B, B - Health
LEFT, RIGHT, UP, DOWN, L, L - $15,000 Cash
LEFT, RIGHT, UP, DOWN, A, R - Lower Wanted Level
LEFT, RIGHT, UP, DOWN, R, A - Raise Wanted Level
LEFT, RIGHT, UP, DOWN, R, R - Wanted Level Max or Zero
LEFT, RIGHT, UP, DOWN, B, R - Toggle Gang Hostility

Level select:
At main menu press: Left, Right, Up, Down, L, R, Hold start and press A. This will unlock a option that displays all the levels.

Get Police Bribes at Hideout:
Complete 10 Vigilante Missions to get one police bribe at your hideout. Complete 20 on each island to get six police bribes at your hideout.

Never get tired when running:
Complete 12 Paramedic missions to never get tired when running.

Package Rewards:
As you collect the hidden packages, weapons will be delivered to your hideout.

Handgun - collect 10 packages
Unzi - collect 20 packages
body armour - collect 30 packages
shotgun - collect 40 packages
molotovs - collect 60 packages
grenades - collect 50 packages
assualt rifle - collect 70 packages
mini gun - collect 80 packages
rocket laucher - collect 90 packages
$1,000,000 - 100 packages

Mission Rewards:
Flame Thrower - Complete 20 Fire Truck missions on each island

Health Pickup at hideout:
Complete 9 paramedic missions

Unlockable Cars:
F-1 Racer - Complete all 18 race missions. The car will appear at (82, 19) on the last island, where the last set of races began.

Borgnine Taxi - Complete 50 Taxi missions and it will appear at your Portland hideout.

Cruise Control:
While in a car, hold Up and release the gas. Your car will continue at the same speed you were at when beginning the sequence.