I found some cheats after analyzing the game EXE a bit; enter them at the password screen. Note that achievements are disabled when you use these!
Invulnerable - Right, Left, Right, Up, Down, Right, Left, Up, Up, Up, Up, Left, Left, Left, Left
Infinite Lives - Right, Up, Left, Right, Right, Right, Down, Up, Down, Up, Down, Up, Left, Left, Left
Has All Keys - Up, Up, Left, Up, Down, Up, Up, Down, Down, Left, Left, Right, Up, Right, Right
Has All Bonuses - Down, Right, Up, Down, Up, Down, Right, Up, Right, Right, Right, Up, Right, Down, Up
Camera Free Look - Up, Up, Down, Left, Up, Left, Up, Up, Left, Up, Left, Left, Right, Down, Down

There is also a debug menu, enabled by entering the following three codes one after the other and then looking in the settings menu:
1) Right, Up, Left, Up, Left, Right, Right, Down, Down, Left, Left, Right, Down, Left, Down
2) Right, Left, Left, Right, Down, Right, Up, Up, Up, Left, Left, Left, Up, Down, Right
3) Down, Down, Down, Up, Up, Down, Down, Right, Left, Down, Left, Up, Down, Right, Up

Additionally there are two more codes that appear to do nothing:
1) Up, Down, Down, Up, Left, Up, Down, Right, Up, Down, Right, Right, Right, Right, Down
2) Down, Right, Right, Right, Down, Right, Right, Right, Down, Right, Right, Up, Left, Right, Down

I'd like to play around with some of these cheats. Does the game disable achievements only for individual save files, or does it disable them game-wide?

I want to still be able to earn achievements on save files I'm not using cheats on.

Okay, why is camera free look considered a cheat instead of being an option? I can understand achievements being disabled when having infinite lives, but why camera free look?