Unlock Professional Difficulty:
Successfully complete the main game once on Normal difficulty. In Professional difficulty everything is exactly the same except that the enemies are harder to kill.

Unlocking the Punisher Pistol:
Do the secret quest the man who sells you upgrades for weapons gives you. You have to search 2 areas of the game for the blue medallions and shoot ALL 15.

Unlock Rocket Launcher and the Mitilda Handgun:
Clear the game on Normal and you get a chance to buy the Rocket Launcher and the Matilda. The Rocket Launcher is $1,000,000 and the Matilda (which fires 3 round bursts) is $70,000.

Unlock Hand Cannon Weapon:
On the Mercenaries mini-game get a five star ranking.

Unlock Assignment Ada Mini-game:
Beat the game to unlock the mini game

Unlock Hunk in Mercenaries:
Get a 4 star rating on the Island Commando map of the game.

Unlock Ada Wong in Mercenaries:
Get a 4 star rating on the Pueblo map of the game.

Unlock Albert Wesker in Mercenaries:
Get a 4 star rating on the Waterworld map of the game.

Unlock Jack Krauser in Mercenaries:
Get a 4 star rating on the Castle map of the game.

Unlock Matilda Weapon (which fires 3 round bursts):
Successfully clear the game.

Unlock Hand Cannon Weapon:
Get the five star ranking on the mercenaries mini-game.

Unlock Rocket Launcher:
Beat the game on Normal and you get a chance too buy the Matilda ($70,000) and the Infinite Launcher ($1,000,000).

Unlock S.T.A.R.S. Uniform:
Complete The Game On Normal Or Hard Mode.