As of season 6 episode 'The Third Man', the amount of weapons in the trunk has increased vastly, since all of Sam's were moved from his car to the impala after Castiel and an unnamed angel flattened it. Weapons seen in the trunk include:
•Hand guns
•Shotguns
•Rifles
•Knives
•Stakes
•Holy water
•Holy oil
•Gasoline
•Salt
Also seen in trunk, but not used on screen include:
•A 37mm DefTech Grenade launcher
•Brass knuckles
•A harpoon gun
•Axes
•Nunchucks
The trunk also has a Devil's Trap on it for effectively transporting demons withing the impala.
