To unlock the Infinite Rifle, unlock the UFO ending. To unlock the UFO ending, use Room 502 Key at the motel room.

Complete the game to unlock the following options:

Walk/Run (toggles running or when Square is held)
Extra Blood (creatures more bloodied when attacked)
Noise Filter (sharpens the graphics)
Bloody Footprints (always leave bloody footprints)
Torch Projection (project image of smiley face, soccer ball, jack-o-lantern, or the mark of Samuel from flashlight)

At the start of the game in the hospital, there is a box with a heart inside. You need a password to open it. The clue is "Remember, the three beauties who cause it to pump so are the key, 'age before beauty'". The code is "23, 31, 19".

Unlock the following bonuses by performing the corresponding tasks:

Ambassador Accolades (Tesla Rifle, Space Outfit costume, and UFO ending)
Complete the game. Then, pick up "Room 502 Key" on the stairs at the beginning of the next game, and use it in the motel.

Brawler costume
Kill over 50% of enemies with only your fists.

Butcher Accolade (Great Cleaver weapon, Butcher costume that grants immunity from Straightjacket acid spit, and Bad ending)
Complete the game. Then, start a new game from your cleared saved game, and accumulate 100 kills. At the end of the game (not the UFO ending), you will see what Travis has become.

Cartographer costume
Check the map less then 25 times.

Collector costume
Collect more then 300 items.

Daredevil
Complete the game without using any saved games.

Explorer costume
Run or walk over 22.5 km.

Fireman Accolade (Fireman's Axe weapon and Fireman Costume)
Rescue Alessa from the fire at the start of the game in under 80 seconds.

Konami Code Accolade (Vincent from Silent Hill 3 costume)
At any time during your second or later replay, press Up(2), Down(2), Left, Right, Left, Right, X, Circle.

Savior Accolade (Lonely Moon gauntlets weapon, Lonely Moon Leather costume, and Good ending)
Complete the game with the Good ending. The Good ending is the default ending unless you qualify for the UFO or Bad ending.

Savior costume
Complete the game.

Sharpshooter costume
Kill over 75% of enemies with a firearm.

Sprinter Accolade (Fitness Costume that grants Unlimited Fatigue)
Complete the game in under two hours.

Stalker Accolade (Night Vision Goggles and Black Ops costume)
Have the flashlight on less than three hours during the game.

Weaponsmith costume
Kill over 75% of enemies with melee weapons.

Complete the game to unlock the Moon Gauntlets.

Unlock the following ending by performing the corresponding tasks:

ENDING - HOW TO UNLOCK
Bad - Kill 200 enemies during your second playthrough.
Good - Complete the game.
UFO - Use Room 502 Key at the motel room.

Unlock the Night Vision Goggles, and have them equipped. In the Exam Room of Alchemilla Hospital is an X-ray screen with "Sarah 19" written on it. Examine the X-Ray while wearing the Night Vision Goggles. You will see an image of the Goggles on the screen. The image changes location, and seemingly its shape, if you alter your position in front of the X-ray before examining it.