Beat the game with Jill or Chris and save. You'll get a new background on the main menu. Choose "Once Again" and you'll find that you can now choose a new difficulty setting.
Beat the game with Jill or Chris and save. You will get a key. Choose the "Once Again" option, then enter the room with the large mirror on the second floor of the mansion. Unlock the door in the back. Now walk to the end of the clothes rack and you'll find another outfit. You can get yet another outfit if you beat the game a second time.
Samurai Edge weapon:
Beat the game with Jill or Chris in less than five hours. Save the game, then start again to begin with the Samurai Edge weapon.
Real Survivor, more costumes and weapons:
Beat the game in "Once Again" mode on Normal in less than five hours. This unlocks the "Real Survivor" option, the .40 Smith and Wesson with infinite ammo and extra costumes.
Rocket Launcher with infinite ammo:
Beat the game with Jill or Chris in less than three hours. Save the game, then start again to begin with the Rocket Launcher with infinite ammo.
Beat the game twice with Jill or Chris to unlock the "Invisible Enemy" option. All enemies will be transparent.
One Dangerous Zombie:
Beat the game with both Jill and Chris to unlock the "One Dangerous Zombie" option. One zombie will follow you around - kill him and it's game over.
Beat the game in "Invisible Enemy" mode in less than five hours to unlock a "Special Features" option that displays a message from the developers and a gallery.
Grenade launcher cheat:
You must have the grenade launcher and two types of ammo (e.g., Grenade Rounds, Incendiary Rounds). Put all items in item box. Put the Grenade Launcher in upper-left item slot. Put Incendiary Rounds in upper-right item slot. Close box. Open inventory. Equip Grenade Launcher. Open box. Select Incendiary Rounds from inventory. Press 'A' twice. Move cursor to Incendiary Rounds inside box . Press 'A' twice.
The amount of Incendiary Rounds in the Grenade Launcher will now increase. The number value will be graphically glitched. Close the box and open inventory to fix that, and enjoy the tons of ammo.
Thanks to Revolution readers Kurt Hamilton, BobSherryIII, Blade Omen, Justin Wagers, Chris Weese, joe, Jeffrey Wanakamik, Mark, and rafael moriel!