Ok back in the day when most of us did not have the net you would stumble upon secretes in games.
So I will start:
MK 2 Fighting smoke after doing the right kombo of buttons on the portal level with toasty popping out. Did not know what the kombo was but I did it by mistake and started fighting smoke. This was on the arcade version. I was so amazed.
MK 3(original not ultimate.)
There was a endurance mode hidden in mk 3. All you had to do was hold down all the buttons on each sega controller and press start. It was in debug mode and only on the subway and street level. Also you would choose six(I think might have been eight)characters and you would duke it out. Was fun.
Dark forces 2 I found sam and max in a room. Don't remember what level it was.
Quake 1 Nightmare mode was hidden on hard difficulty. You had to go to hard on the entrance hallway. then down a magical pond or something like that.
Serous sam to many to list.
Some where in unreal 2 I found a turning box with photos of the development team.
Duke3d we all know those ones. finding the doom marine, terminater,Indiana Johns,strippers in a cave,Alien,Simpsons,Star trek and so on.