Post the game, system, and the cheat code (What it does and how to do). Glitches do not count (leave that to another thread). GameShark codes for old systems are acceptable too. So a cheat code I remember is:

Twisted Metal Black {PS2}:
God Mode (Invincible, Unlimited ammo,etc.): Hold shoulder buttons {L1, L2, R1, R2} Up, X, Left, O

Perfect Dark {N64} [GS]:
All Cheats Unlocked (Zelda Keycode Required): 50004002 0000, 810A2218 0001