Easy Hits:
At the beginning of the game when you get the stick with nails on the end (first weapon), run around your enemies and hit them once. They are very slow and will

Die In 1 Hit.
Chainsaw:
Find the hidden Chainsaw weapon. Once you beat the game on Normal with Normal puzzle difficulty, start a new game and the chainsaw will be hidden in the logs before the cemetary.

Extra Mode:
Beat the game for more options. Once you complete the game, you can find several new options in the Options menu, such as Bullet Adjust (2 or 3 times the normal ammo supply), and an option that allows you to deactivate the noise filters in some scenes.

Extra Puzzle Difficulties:
Once you've beaten the game using the Easy, Normal, and Hard puzzle settings, you will find that playing a new game under Hard mode gives you new puzzles.

Hyper Spray!:
Another new weapon? Beat the game twice, then start a new game. Enter the motor home on the southern side of the town and nab the Hyper Spray.

Visible Signs:
Reveal All Hidden Signs. Once you've opened up all five of the game endings, you can begin a new game and all the signs will be viewable.

Options menu:
In the options menu, press L1 or R1 for extra options. These options allow you to change the blood color, whether or not your map is always zoomed in etc.