Undertale

Easter Egg Passwords
If you try to enter the following names as your character you will get the corresponding extra hidden dialogue. These names unless otherwise stated cannot be entered as your name in the game.

Asriel:
'...'

Catty (can use as own name):
'Bratty! Bratty! That's MY name!'

Alphys:
'D-don't do that'

Undyne:
'Get your OWN name!'

Temmie (can use as own name):
'hOI!'

Flowey:
'I already CHOSE that name'

Toriel:
'I think you should chose your own name my child'

Bratty (can use as own name):
'Like, OK I guess'

Sans:
'nope'

Murder (can use as own name):
'That's a little on- the-nose, isn't it...?'

Mercy (can use as own name):
'That's a little on- the-nose, isn't it...?'

Chara (can use as own name):
'The true name'

Asgore:
'You cannot'

Gaster:
(Reloads intro sequence)

Weaken Asgore:
If you manage to save the pie that is given to you at the start of the game by Toriel and use it during the final battle against Asgore you will weaken his attacks significantly allowing you to defeat him easier.

Easily Defeat Muffet:
You can defeat Muffet in a single turn by using one of the Spider Bakery items such as the Spider Donut and Spider Cider, both of which can be purchased early in the game.

Boost Mettaton EX's Ratings:
You can boost your ratings by default and make the final fight with Mettaton EX easier by having certain items such as the cloudy Eyeglasses, Stained Apron, and Glamburger.

Developers Room:
If you complete the credits without touching any of the names you will be able to open the previously locked large door in Snowdin. Unlocking this door will give you access to the developers room.

Hard Mode:
To unlock this feature enter 'Frisk' (without the quotes) as your starting name. The warning 'make your life hell' will appear on screen to confirm confirm correct code entry. In Hard mode you cannot retreat on the first turn of battles, will face tougher and new enemies, and can only hold three pieces of candy.