Call of Duty 3 Codes -

Level selection
Hold in the Chapter screen and press the following buttons to unlock all levels: x2, x2, x2

Hold Extra Grenades
Normally, you're only supposed to hold 6 grenades but when you have a grenade thrown at you you can pick it up and throw it again. Instead of throwing it you can keep it for later by pressing the minus button a second time after picking up a grenade. There's no limitation of grenades when doing this.