Mortal Kombat Trilogy
Play As Galaga or Pong
Select 8-Player Kombat from the Mode Select Screen and win the whole thing. You will now be able to select one of SHAO KAHN'S LOST TREASURES. Select the second symbol (MK) on the first row to play Galaga, and select the first symbol (Rayden) on the second row to play Pong.
Credit Goes To: Frank Marano via N64cc

 Play As Galaga or

 Ultimate Kombat

 Play Space Invaders
 Select Your Battle

 Play As Shao Khan
 Play As Motaro
Ultimate Kombat Kode
To enter an Ultimate Kombat Kode at any time, simply rotate the control pad or analog control stick a couple of times clockwise at the Story screen or during a demo. It will go to to the Kombat Kode screen as if you had just beat the game.

033-000 Player 1 handicap
000-033 Player 2 handicap
460-460 Randper
788-322 Fast uppercut recovery
044-440 No power
300-300 Silent Kombat
100-100 Throwing Disabled
010-010 Throwing Encouraged
020-020 Blocking Disabled
205-205 Winner fights Smoke
769-342 Winner fights Noob
969-141 Winner fights Shao Kahn
033-564 Winner fights Motaro
444-444 Randper Kombat
282-282 No Fear
987-666 Flipper message
550-550 Wavenet UMK3 message
999-999 Version Number message
448-844 "Don't jump at me.." 
717-313 Rain is in the Graveyard
122-221 "Skunky!" 
004-400 Ed Boon message
987-123 No Powerbars
688-422 Dark fighting
985-125 Psycho kombat
642-468 Play hidden game
788-322 Uppercut Recovery
466-466 Unlimited running
321-789 Super Run Jumps
975-310 Health Recovery
722-722 Combos Disabled
555-556 Special Moves Disabled
024-689 Super Endurance Kombat
484-484 Automatic Kombos
109-901 Bloody Kombat
202-808 "Babalities Are Reversible"
123-321 Winner Fights Khameleon
246-246 Play Pong
222-222 Dead Pool
191-191 The Armory
919-919 The Pit
606-606 Star Bridge
101-010 The Tower
007-007 The Portal
166-661 The Pit II
121-121 The Courtyard
212-212 The Wasteland
000-666 The Lair
For 2-on-2 and 3-on-3 only: 
022-220 Explosive Kombat/Throwing Disabled
227-227 Explosive Kombat
330-033 Kombat Zone:Jade's Desert
666-444 Scorpion's Lair
091-190 Bell Tower
666-333 Graveyard
933-933 Sizlac Busorez
880-088 Subway
343-343 The Roof
820-028 Pit III
004-700 Kahn's Kave
002-003 River Kombat
050-050 Noob's Dorfen Hidden
600-040 Kombat Temple
079-035 The Street
123-901 The Soul Chamber
077-022 The Bridge 
880-220 Kahn's Tower
Credit Goes To: N64 Cheats & N64 HQ & Brian via N64cc

Play Space Invaders
In a two-player game, select the Pit stage, and if an object like Santa or something else flys over the moon, hit the Z-trigger button. The winner of that round then gets to play Space Invaders. You might have to play the level a few times before something will show up over the moon.
Credit Goes To: Nathan Clevenger via N64cc

Select Your Battle Arena
On the Player Select Screen highlight Sonya. Hold Up on either the control stick or pad and press Start. You will see the screen shake and hear a boom if you did it correctly. After you have chosen your fighter(s) you can choose the stage!
Credit Goes To: N64cc

Play As Shao Kahn
Select any character. You must be on either the Pit 3, Rooftop or Kahn's Kave stages. Before the round starts, hold Down, HP, and LP. You'll see an explosion and Shao Kahn will replace your character. If you win, you will remain as the original character you picked, and not Shao Kahn.
Fatality (Hammer): F, B, B, HP (sweep)

Play As Motaro
Select any character. You must be on either Jade's Desert, Kahn's Tower or the Wasteland stages. Before the round starts, hold Back, HK, and LK. You'll see an explosion and Motaro will replace your character. If you win, you will remain as the original character you picked, and not Motaro.
Fatality (Head Rip): F, F, F, HK (close).
Credit Goes To: Bryan Dawson via N64cc