To active cheat codes you must type www.freespace2.com during game. Note that you cannot progress through a campaign after activating cheats. Cheats provided by Official site.

~ + C		player collision on/off
~ + SHIFT + C		toggles countermeasures on all ships.
~ + K		destroys your target.
~ + SHIFT + K		destroys targeted subsystem
~ + ALT + SHIFT + K		does ~10% damage to your target.
~ + ALT + K		does ~10% damage to yourself.
~ + I		toggles invulnerability.
~ + SHIFT + I		toggles invulnerability on target.
~ + W		gives the player infinite weapons.
~ + SHIFT + W		gives all ships infinite weapons.
~ + 9		scrolls forwards through all secondary weapons
~ + SHIFT + 9		scrolls backwards through all secondary weapons
~ + 0		scrolls forwards through all primary weapons
~ + SHIFT + 0		scrolls backwards through all primary weapons
arrrrwalktheplank		Call in the Volition pirate ship!
tooledworkedowned		Ewwww
vasudanswuvfishes		Enter at Vasudan main hall. FISH TANK!
humanheadsinside		Enter at Vasudan main hall. Ugh!

To active cheat codes you must type www.volition-inc.com during game. Note that you cannot progress through a campaign after activating cheats.

~ + SHIFT+C		toggles countermeasures on all ships.
~ + G		marks all primary goals complete.
~ + SHIFT + G		marks all secondary goals complete.
~ + ALT + G		marks all bonus goals complete.
~ + K		destroys your target.
~ + SHIFT + K		does ~10% damage to your target.
~ + ALT + K		does ~10% damage to yourself.
~ + I		toggles invulnerability.
~ + SHIFT + I		toggles invulnerability on target.
~ + 0		toggles Descent-style physics.
~ + R		requests rearm for target/player (if no target selected).
~ + W		gives the player infinite weapons.
~ + SHIFT + W		gives all ships infinite weapons.
~ + 9		scrolls forwards through all secondary weapons.
~ SHIFT + 9		scrolls backwards through all secondary weapons.
~ + 0		scrolls forwards through all primary weapons.
~ + SHIFT + 0		scrolls backwards through all primary weapons.