Type any of the following codes on the keyboard while in any of the game's front-end menus. A notification popup should appear on screen notifying you of a successful cheat input. CHEATS can be turned on and off, and must be reactivated whenever the player returns to the main menu after a game-over (though will stay activated if selecting Play Again from Game Over).
FunkyFace - Modifies the bottom-screen text notifications in game
OmNomNomable - Changes "Nyans" to "Noms" in gameplay
VertigoKitty - Flips the galaxy upside-down
ColdWintersDay - Makes Nyan Cat move slowly from lane to lane
DownInFront - Moves Nyan Cat far up the screen
SnookiesCookies - Changes the word "cookies" to "Snookies" in the text notifications
ReverseEngineering - Reverses the galaxy horizontally, and inverts the rainbow trail
TastingTheRainbow - Reverses Nyan Cat
NyanCatMarch - Replaces bottom-screen text notifications with a parade of Nyan Cats
UseYourWhiskers - Disables the HUD
Leprechaunitis - Disables Nyan Cat's rainbow
CatBreadForTheWin - Replaces all yummies with Cat Bread!
InvisibilityCloak - Makes Nyan Cat completely invisible
OUTFITS immediately change the appearance of the Nyan Cat itself.
BlackAlbum - Makes Nyan Cat completely black
WhiteAlbum - Makes Nyan Cat completely white
DreamsComeTrue - Makes Nyan Cat flash rainbow colors
SummerNyan - Turns Nyan Cat into a watermelon
ChocoKittyTart - Makes a Chocolate Pop-Tart Themed Nyan Cat
YouLookLikeAPopTart - Makes Nyan Cat white, pink and purple
NyanOLantern - Makes Nyan Cat Hallowe'en-Themed
NyanFisher - Makes Nyan Cat stealth themed (a la Sam Fisher)
GGGGhostNyan - Makes Nyan Cat transparent
NyanTheDarkLord - Makes Nyan Cat black and glow red. Red glow is retained after reloading the game, black is not.
NyanStamp - Embosses Nyan Cat
ForgotMyGlasses - Makes Nyan Cat blurry and nearly invisible
DivideByZero - Makes Nyan Cat invert the color of whatever is behind it