Chrono Trigger Codes -

Alternate endings:
Complete the indicated task to view the corresponding alternate ending.

Beyond Time: Defeat Lavos after reviving Crono at Death Peak. 
Dino Age: Defeat Lavos after facing Magus, but before facing Azala. 
Dream's Epilogue: Defeat the Dream Devourer beyond Time's Eclipse. 
Good Night: Defeat Lavos after visiting the End of Time, but before returning to the Middle Ages. 
Memory lane: Defeat Lavos after Schala opens the sealed door, but before restoring the light to the pendant. 
People Of The times: Defeat Lavos after regaining the gate key, but before giving the Masamune to Frog. 
Reunion: Defeat Lavos while Crono is dead. 
The Dream Project: Defeat Lavos in the Ocean Palace. Also available after starting New Game + mode. 
The Legendary Hero: Defeat Lavos after arriving in the Middle Ages, but before obtaining the Hero's Badge. 
The Oath: Defeat Lavos after giving the Masamune to Frog but before fighting Magus 
The Successor Of Guardia: Defeat Lavos after saving Leene and Marle, but before visiting the End of Time. 
The Unknown Past: Defeat Lavos after obtaining the Hero's Badge, but before the Gate key is stolen. 
What The Prophet Seeks: Defeat Lavos after facing Azala, but before Schala opens the sealed door. 

New Game+ mode:
Defeat Lavos and view any of the endings.

Dimensional Vortex dungeon:
Defeat Lavos at the end of Blank Omen and complete the game.

Lost Sanctum dungeon:
Unlock the Black Omen in 12,000 B.C. The Lost Sanctum dungeon is now accessible in 65 million B.C., and 600 A.D. at certain locations.