GTA Weapons & Power Ups

Machine Gun
Rocket Launcher
Flame Thrower

Power Ups

Get Out Of Jail Free Card -  After being caught by the police you retain all of your inventory and your multiplier is preserved (Otherwise your multiplier is cut in half and you lose all weapons)
Police Bribe - Stops the cops from chasing you
Armor - Protects you from gunfire (good for 3 hits)
Extra Life - Fairly self explanatory
Kill Frenzy - Gives you a weapon with unlimited ammo for a limited period of time
Car Speed Up - Take a wild guess
Multiplier Up - Raises your multiplier by one
Info Box - Not really a power up, but they are handy when your first start playing