While playing the game, type www.freespace2.com.
Message will show "Cheats activated." Then hold down the tilde key (¬) and press any of the following keystrokes to activate the cheats:

C = Send message to enemies.
SHIFT-C = Toggle availability of countermeasures for all ships.
K = Kill target.
SHIFT-K = Destroy targeted subsystem.
ALT-K = 10% damage to yourself.
I = Invulnerability.
SHIFT-I = Toggle invulnerability for target.
O = Toggle Descent-style physics.
SHIFT-W = Infinite weapons for ALL ships, including yours.
W = Infinite weapons for just YOUR ship.
G = Mark all primary goals complete.
SHIFT-G = Mark all secondary goals complete.
ALT-G = Mark all bonus goals complete.
9 = Scroll forward through all secondary weapons.
SHIFT-9 = Scroll backward through all secondary weapons.
0 = Scroll forward through all primary weapons.
SHIFT-0 = Scroll backward through all secondary weapons.
R = Issue a re-arm request for target.

Note: You CANNOT advance the mission if you activate the cheat codes, unless you do the following trick:

Go into one of the single missions and activate the unlimited ammo cheat. Leave the single mission (you don't have to finish) and return to the campaign. You will now have unlimited missiles and keep the ability to advance in the campaign