PlayStation: Start a 2 Player game and at the versus screen press these buttons to enter a kombat kode: (Player 1) Low Punch x3, Low Kick x2 - (Player 2) Low Punch x2, Block, Low Kick x3. After the match starts, pause and exit the to the main menu, select Options, highlight "Vs Screen Enabled" and hold Run + Block. Hold the buttons down until the Cheats menu appears.
PC: Go to the options menu and highlight Continue, Hold Low Kick + Block, the cheat menu will appear after you hear "Outstanding!".
Nintendo 64: Same as above, but holding the buttons Block + Run instead.

Noob Saibot: Beat the game with Reiko. Then, at the character select screen, choose the "HIDE"(?) box, and then select Reiko while holding down Run+Block until the fight begins.
Goro: Beat the game with Shinnok. Then, at the character select screen, choose the "HIDE"(?) box, and then select Shinnok while holding down Run+Block until the fight begins.
Meat: To get to play as Meat, the bloody skeleton, you much select Group mode and win all the way through it. Once done, you then get to choose a character at the select screen. During gameplay, you will have a bloody skeleton with the moves of the character you chose. You continue to play with him until you lose.