Yes, I know most people would think of the famous Konami Contra code when they see the topic. However, there are some other codes which are pretty memorable.

Some I remember:

Street Fighter II (SNES): Same character code. Kinda dumb while looking back at the time when 2 players cannot choose the same fighter.

Sonic 2 (Genesis): Ability to go super saiyan jin and unique way of entering codes through the sound test options

Dragonball (SNES): Enabling secret characters. Another unique way of entering the code via clockwise and anticlockwise rotations.