Alternate realities, aka the different endings
One of the most unique aspects of the Chrono games is the ability to find alternate endings by completing the final battles at inappropriate times in NEW GAME + mode - effectively, by defeating the final threat before bringing each story its necessary closure.
A. CHRONO TRIGGER ---------------------------------------------- Note: To discover the extra endings in New Game + mode, use the extra Time Gate which appears between the two pads of the Telepod in Leene Square at the Millennial Fair. You can also defeat Lavos in the Ocean Palace in 12,000 B.C. to acquire one of them - definitely the most difficult approach, as Lavos' attack at Queen Zeal's behest was designed to be a deadly no-win battle, and the task of simply breaking Lavos' shell takes an exceptionally long time thanks to its tremendously high HP. The other means of defeating Lavos - using the bucket at the End of Time or the Wings of Time - will lead to the "proper" ending.
Some of these endings contradict the actual story, so don't take them too seriously. It's all in good fun. So please, refrain from launching tiresome screeds against Square because the Reptites would have died anyway thanks to Lavos' impact on the planet or because Crono, Lucca and Marle were bad guys or something.
-- 0a. GAME OVER, MAN! ------------
REQUIREMENTS: Get your butt whomped by any enemy and die.
EVENT: The future refuses to change....
-- 0b. NO CRONO ENDING ------------
REQUIREMENTS: Defeat Lavos by using the conventional means (the Wings of Time, the Lavos bucket in the End of Time, etc.) before resurrecting Crono at Death Peak.
EVENT: ???
-- 0c. TRUE ENDING ----------------
REQUIREMENTS: ???
EVENT: ???
-- 1. DEVELOPER'S ENDING ---------
REQUIREMENTS: Travel through the Gate at the Millennial Fair and defeat Lavos with only Crono and Marle before Lucca's Telepod is set up at the beginning of the game. This is not an easy task in your first playthrough, so you may want to wait a while. Alternately, you can see this ending by defeating Lavos in 12,000 B.C. when Queen Zeal summons it, although this is arguably even more difficult than using a two-person team.
EVENT: The victorious party is given a chance to meet the development staff of Chrono Trigger and putz around for the heck of it. Quite amusing in places.
-- 2. AMPHIBIAN LOVE ENDING ------
REQUIREMENTS: ???
EVENT: ???
-- 3. WASTE OF MEMORY ENDING -----
REQUIREMENTS: ???
EVENT: ???
-- 4. YAWN-TASTIC ENDING ---------
REQUIREMENTS: ???
EVENT: ???
-- 5. THE "UH, WHAT?" ENDING -----
REQUIREMENTS: ???
EVENT: ???
-- 6. THE GURUS ENDING -----------
REQUIREMENTS: ???
EVENT: ???
-- 7. LONE WOLF, ER, FROG ENDING -
REQUIREMENTS: ???
EVENT: ???
-- 8. DAMN DIRTY DINOS ENDING ----
REQUIREMENTS: ???
EVENT: ???
-- 9. UNFINISHED BUSINESS ENDING -
REQUIREMENTS: ???
EVENT: ???
-- 10. LADIES' NIGHT ENDING ------
REQUIREMENTS: ???
EVENT: ???
B. CHRONO CROSS ------------------------------------------------ Note: To acquire all "extra" endings, use the Time Egg at Opasa Beach and destroy the final boss rather than Chrono Cross-ing it (obviously, since you won't have the Chrono Cross for most of these).
-- 1. DEVELOPER'S ENDING ---------
REQUIREMENTS: Defeat the TimeDevourer with Serge and no one else. Not even Poshul. You absolutely must equip the Green Plate armor and a Sea Charm to prevent being slaughtered by Omega Green or being afflicted with Freeze status (an instant game-over for a one-man party). A Spectra Swallow is a must-have as well; equip lots of high-level attack Elements and make the most of whatever Field Effect advantage you can acquire.
EVENT: Reminiscent of the programmers ending in Chrono Trigger. I won't spoil it; have fun exploring.
-- 2. THE ACACIAN EMPIRE ENDING --
REQUIREMENTS: Defeat the final boss with a multiple-character party prior to choosing a companion with whom to infiltrate Viper Manor.
EVENT: Serge returns to Arni and becomes a fisherman, and he and Leena are pretty well on their way to wedded bliss (or hell, whatever). Kidd appears briefly to encourage Leena and wish Serge farewell, then vanishes. She reappears at Viper Manor, where she singlehandedly defeats Lynx and steals the Frozen Flame, wishing herself lord of the manor. Taking charge of the Acacian Dragoons, Kid leads the nation of El Nido into a the era of the Acacian Empire.
As a side note, this ending mirrors a small bit of the trailer movie that was shown at the end of the Japanese demo. In it, General Viper (not Kid) was the one who declared war on Guardia and Porre. This bothered me for a long time, because every other scene shown in the demo appeared in the finished game in one form or another. So this brings a sense of closure to the demo/game relationship.
3. WACKY FUN ENDING
REQUIREMENT: Defeat the boss after reaching Guldove, before Kid is healed. If Nikki and Razzly have been recruited, your ending will be different.
EVENT: Serge and Lisa from Termina are now an item, much to Leena's dismay. And for a friend, Lisa's awfully backbiting... At Viper Manor, Norris the spy reveals himself to be the mysterious benefactor who gave Kid the Hydra Humour she needed to recover, placing Kid in his debt. Meanwhile, Peppor and Solt have taken matters in their own hands to hunt down Lynx, who has disgraced the Acacians. They hire Pierre, who, alas, is less than the ideal for the task, what with his terrible case of "the squirts"...