Some cool stuff I found.
Mortal Kombat 2 arcade game:
Hold L2 and press Square, Up, Down, Right, Left, R2, Square.
-OR-
Complete all five Smoke missions.
Play as Scorpion in 1 player:
Hold L2 and press Square, Up, L1, R1, Left, Right, Square.
-OR-
Complete the game with Liu Kang.
Play as Sub-Zero in 1 player:
Hold L2 then press Square, Down, Up, *L1, Up, Square.
-OR-
Complete the game with Kung Lao.
*if the code doesn't work, try hitting L2 instead.
Play as Scorpion in VS:
In the Foundry area, follow the pathway downstairs to the left of the save point. Two archers will break through the wall. Throw one at the wall to the left of the doorway to break it open, giving you access to a hidden room full of lava and a koin on a ledge. Grab one of the archers and throw him into the lava. Then, double jump off his back to get it.
Play as Sub-Zero in VS:
Take the boat ride across blood lake. The Koin should be behind the lion-like statue to your left. Run along the wall and double jump at the end to collect it
Play as Baraka in VS:
In the Soul Tomb Portal, go to the area where you must wall climb. You will see a path that leads to the left and one that leads to the right; take the path to the right. There will be a statue on the path; use the Fist of Ruin to break it. Continue along the path and use the long jump ability to cross the gap and get the Koin.
Play as Johnny Cage in VS:
In the area that is on fire, go to the extreme right of the gate, then jump onto the roof of the house. Run to the right and jump to reach a new platform with a boulder blocking your way. The koin should be right there.
Play as Kitana in VS:
In the Evil Monastery (just before the portal with the two statues in front of it), you will find a number of pillars. Jump on the half broken pillar, then double jump on the roof. There should be a koin.
Play as Noob Saibot in VS:
In Sub-Zero vs. Sub-Zero matches, Player one will be Sub-Zero, and player two will be Noob Saibot.
Play as Reptile in VS:
Go to the small room with the waterfall in Living Forest. Long Jump across waterfall, then go through the door. Throw the clay statue into the giant stone column to your right, where you entered. It should fall, and a koin will appear in the previous room in the waterfall.
Easy health:
*In 1 player, whenever you meditate (even if you don't save) your health should be fully restored.
**In Co-op, whenever you die, when you come back into the room, your health should be fully restored.
*This doesn't work in Co-op.
**This doesn't work in 1 player.
Easy experience:
In Goro's lair, in the room where you have to break the wall, just keep killing the enemies instead.
-OR-
Press start whenever you hear 'toasty' to get an extra 1000 experience.
Easy Test Your Might:
Press Square with X to make the bar go up twice as far.
Fight against Ermac:
*Go to the Wu-Shi Academy and break the statue to the left near the beginning. Then long jump across the pit, and enter the Warrior's Shrine (AKA Hall of Champions) and wall run up the wall to your left. There should be a small ledge at the top with a bunch of poles. Use the swing to get to the other side and examine the statue. You should now have access to the Shrine. Enter and examine the Sub-Zero/Scorpion statue three times and Ermac should appear.
*You must have Long Jump, Fist of Ruin, Wall Climb, and Swing to be able to do this.
Fight against Kano:
Before you fight Shao Khan, go to your right and talk to Kitana three times. A few guards should break through the door and fight. Ignore them and enter the door. You will now have to fight Kano (Jax is on your side). Once he's dead Jax should give you a health upgrade, and during the fight with Shao Khan, a medium size health should appear where Sonya was supposed to be.
Fight against Mileena
*In the Living Forest, swing near the first save point into a tree. Climb up to find a secret area, and at the end you will fight Mileena.
*You must have Wall Climb and Swing to be able to do this.