Hidden Codes:
First, enable the cheat menu. At the title screen, quickly enter the following:

Up, Up, Down, Down, Left, Left, Right, Right

You'll hear a confirmation sound if done quickly enough. Press Start to access the main menu, and then press:

Block + Run

You'll access the cheat menu.

When you get to the character selection screen, highlight "Hidden", and put in the code you desire.

Hold L + R, and press Up, Up, Left, Left, Left, A - Noob Saibot
Hold L + R, and press Up, Up, Up, Up, Left, Left, Left, Left, A - Sektor
Hold L + R, and press Up, Left, A - Goro
Hold L + R, and press Up, Up, Up, Up, Left, Left, Left, Left (Then play as Sektor until you beat Shinnok.) - Get Sektor's Movie Sequence (Does not work for Goro or Noob Saibot.)

Kombat Kodes:
Input these codes at the versus screen.

044-044 - The Tomb
033-033 - The Elder Gods
022-022 - The Well
055-055 - Wind World
066-066 - Reptile's Lair
101-101 - Shaolin Temple
011-011 - Goro's Lair
110-110 - No Throws & Max Damage
001-001 - Unlimited Run
010-010 - Disable Max Damage
202-202 - Living Forest
303-303 - The Prison
313-313 - Ice Pit
100-100 - Disable Throws
666-666 - Silent Kombat
002-002 - All Time Weapons
050-050 - Explosive Kombat
555-555 - Many Weapons
321-321 - Big Heads
012-012 - Noob Saibot Mode
222-222 - Random Weapon
111-111 - Free Weapon
123-123 - No Power
020-020 - Red Rain
444-444 - Armed & Dangerous

Alternate Costumes:
On the character select screen, highlight a character, hold Start and press any button once to rotate its select screen picture and enable its second costume, and twice for its third. This works with most of the fighters. You may have to press a button a third time to get some characters' costumes.

View Character Bios:
At the main menu select options,then select Kombat Theater.Use the directional pad to highlight a clip then simultaneously press and hold L and R to view the characters Bio screen.