R1, R2, L1, R2, Left, Down, Right, Up, Left, Down, Down, Down

That one was my favorite

Good 'ol MK1 blood code. :D That takes me back.

The cheat code that is forever burned into my memory is the Sonic 2 stage select and debug code: go into the sound test, and play BGMs 19, 65, 9 and then 17. When you hear a chime, go back to the main menu and back to the sound test, and it will be replaced with a stage select.

Can't believe I'm the first to mention (in my mind) the most famous of all, the Konami Code:

up, up, down, down, left, right, left, right, B, A, , *Select, Start

* if i remember correctly you added the select into most games when playing two player.

I did see someone mention Contra but they didn't give the code.

Oh yeah has anyone gotten this code to work on Wii U virtual console games, I've tried it on several without success ...

L+Down, R+C-Right, R+C-Up, L+Right, L+C-Down, R+C-Up, L+Right, R+Down, L+Left, L+R+C-Right

The codes in dark forces lapostal for weapons lapogo to be able to get to high places laskip to skip the level
Lost Vikings I remember several of the level paswords don't know if that counts
The code for skipping a level in Aladdin

Sonic the hedgehog, at the title screen press up, down left, right, A, start. You will hear a kerching and if my memory serves me correct you have a stage select. I once managed to get a semi playable hidden palace stage on Sonic 2 using an action replay cartridge.

none

but I know all the combos in tekken tag tournament and its been 15 years.

Donkey Kong Country 3: L-R-R-L-R-R-L-R-L-R

MERRY - Christmas-themed bonus
WATER - Secret waterfall cave, all bonus coins
COLOR - Change characters colors
MUSIC - Sound test


Yoshi's Island:

Hold select and press X-X-Y-B-A to play minigames.