GTA, Tony Hawk, and a few others!

Mortal Kombat (Mega Drive) A, B, A, C, A, B, B to unlock blood mode.

↑↑↓↓←→←→BA, pretty obvious
How do I turn this on was great fun, summon a Cobra Car with a machine gun to fight your enemies in AoE 2.

Others I knew are.

WB2&pARAcnOwnU
jMCK%hTkJHyrT

You could use it 3 times per day in Animal Crossing to get up to 90K bells.

CIRCLE, CIRCLE, L1, CIRCLE, CIRCLE, CIRCLE, L1, L2, R1, TRIANGLE, CIRCLE, TRIANGLE

My favourite Vice City code.

The Konami 30 Life Code: Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.

The Sega Genesis Mortal Kombat Blood Code: A, B, A, C, A, B, B.

This one possibly may not be a cheat code, but I remember the Mew Glitch in Pokemon Red, Blue, Yellow.

The Konami Code, mostly because it's the only cheat code that I've used more than once!

Black Death, instant victory in Age of Empires II.

FMNFB from Turok: Evolution

And a pile of Age of Empires 2 & Cossacks: European Wars ones

Hesoyam :D

Contra infinite lives code is the only code that i still remember.

I only really know a few Sims cheat codes, but that's it!