SiN
Activision
Access the console by hitting that magic ~ key, then enter these codes: 
	BigfootGold	Win with gold victory
	/health 999	Health 999
	/wuss	All weapons
	/superfuzz	God mode
	/nocollision	No clip mode
	/wallflower	No target 
/spawn rockets	Gives you rockets
/spawn magnum 	Gives you the
			Magnum
	/spawn shotgun	Gives you the
 			shotgun
	/spawn assaultrifle	Gives you the 
			assault rifle
/spawn rocketlauncher	Gives you
	the rocket launcher
/spawn sniperrifle	Gives you the 
			sniper rifle
	/spawn heligun	Drops the big gun 			from the chopper 
		into your arms
/spawn reactiveshields  Gives you a 
		reactive shield
	/spawn coin	Gives you a quarter
	/spawn health	Gives you a health 
		pack
	/spawn cookies	Gives you some
		cookies (mmm)
	/spawn lensflare	Weird light-effect
And if you want an extremely cool secret weapon, go to the console and type 'Give Thrallgun' and then bind it to an easily accessible key; for example: bind M 'use thrallgun'. It gives you two different firing modes which really kick ass.