Cats
It's a fairly well known fact that Crono can win another cat by playing the 80 point game at the fair. But did you know if you keep winning cat food (to win cat food, win a game you already have the prize for. The 80 point game gives the most), you'll eventually end up with more cats! I'm not exactly sure how much cat food you need, but everytime you get somewhere around 60 ounces of cat food, another cat will appear in Crono's room! If you want to know how much cat food you have currently, check the pot/barrel thing in Crono's room.

Clones
We all know you can win a Crono clone from the fair (hell, we gotta do it just to save Crono). Well, after Crono's brought back, you can win yourself another Crono clone. Big deal you say? Well, try switching your lead character. Winning with each character will win you that character's clone. They can be found in the following locations:
Crono-Crono's House (duh)
Lucca-Lucca's House
Marle-Marle's room in the castle
Frog-Frog's house in the Cursed Woods
Robo-Proto Dome
Ayla-Ayla's Hut
Magus-North Cape

Poyozo Dolls
Similar to the Clones, each character can win themself a Poyozo Doll by playing the 10 point game. So what, you ask? I think it's neat, and each doll plays its own song! Each doll will play that person's theme song, except Crono and Lucca's will also play Marle's Theme...

Increase Your Race Odds
You can win some easy silver points by guessing who'll win the race at the fair, but sometimes it's difficult to pick a winning runner! This trick can increase your odds of success. By running alongside a runner, sometimes they'll speed up, so you can run along with the runner you picked, and hope it inspires him to win! Or if you want to slow down a runner, walk alongside him and keep pressing 'a'. If you do it right, he'll slow down to walking speed. Now you can sit back and watch the silver points roll in.

Shrine in the Cathedral
When you reach the first set of stairs in the Cathedral, go up the right staircase, and enter the first room you come to. Then head right to a hidden room, where monsters will be worshipping a statue of Magus. Also, there are some pretty nifty items here.

Visit Your Father!
If you visit Taban with Lucca in your party, periodically he will give you the Taban Vest, Taban Helmet, and Taban Suit, three good pieces of armor for Lucca.

Arris Dome Code
If you are using an emulator and your keyboard to play Chrono Trigger, you know what a pain it is to enter the Arris Dome code (hold L=R and press A). Since you can't enter it conventionally with a keyboard (you can't press three buttons at once), edit your control scheme so that the same button is used for both L and R. Then enter the code (I think you can make L, R and A all be used with the same key as well...)

Jet Bike Prizes
By getting certain scores in the Jet Bike race, you can net yourself certain prizes, but only if RX-XR is there (get the Race Log)
777-10 Mid Ethers (untested)
1300 and up-5 Mid Tonics (it says Mid Ethers but it's a typo)
1500 and up -Power Tab (first time)
2000 and up-5 Ethers(untested)
2300 and up-5 Full Ethers (it says Ethers, but you receive full Ethers...and it also says your score is 2371, which is a perfect score)(untested)
There may be more...if so email me

Fleaver
Searching high and low for the Fleaver? Don't. Its name was apparently changed to the Slasher at the last moment (kinda funny thing I realized. FLEAver...SLASHer...Flea and Slash...^_^)

Cholesterol
Nothing useful can come of this, but it's still pretty funny. Go to Guardia Castle sometime after you beat Magus (but before you get the Rainbow Shell), and make sure Marle's in your party. Marle and her old man will have a fight, and he'll run up to his room. Go to the Snail Stop in Porre and buy some Jerky. Talk to the king, and he'll start ranting about how you're trying to kill him with cholesterol, and then he'll lock you out of his room! =P

Power 'em Up
Once you power up Marle's pendant, you can open up the black boxes. Whoa, but be careful! Don't just open up any old box! Go to 600 AD, and when you examine the box, it'll ask you if you want to open it up or not. Choose not to, and then open it up in 1000 AD , the contents will power up as follows:
White Vest-White Mail
Black Vest-Black Mail
Blue Vest-Blue Mail
Red Vest-Red Mail
Nova Armor-Moon Armor
Siren-Valkerye
Kali Blade-Shiva Edge
*When you get the contents in 1000 AD, go back to 600 AD and you can get the original contents too!

Sealed Doors
Other than the Sealed door in Keeper's Dome, there are also Sealed doors in Bangor Dome, Trann Dome, and Arris Dome. Behind each is some great stuff!

Sealed Weapons
You know that mysterious place to the northeast of Medina Village? Well, go there with a powered up pendant, and you can undo the seal, and get either the Swallow or the Safe Helmet!

Name Change
So you gave Lucca a nerdy, stupid name, but now you realized that she rules, and you want to give her a more fitting name. Well, once you get past the Ocean Palace, head to the Laruba Ruins in 65,000,000 BC, and talk to the Nu. He'll give you the Silver Rock. Talk to him again and he'll offer to change the lead character's name for free! And if you want to change Epoch's name, head to Keeper's Dome and examine the computer near the entrance.

Hidden Zeal Rooms
There are hidden rooms in Kajar and Enhasa, filled with nifty stuff. Somebody in Zeal tells you "Water summons Wind, Wind makes Fire dance!", but what the hell does that mean? I'll tell you what it means, don't get your pants in a festival (to quote Keenan's partner, Kel). In Kajar and Enhasa, there are three books you can open, a Wind, Fire and Water book. Open the Water book, then the Wind book, then the Fire book, and a hidden passage will be opened up. In Enhasa, this'll lead you to a fight with 6 Nu's, with a Magic Tab and Speed Tab as the reward. In Kajar, you'll win the Black Rock.

Gold Rock
This is a tough rock to get, so I'll tell you how. After powering up the Masamune, make Frog your group leader and go into the Denadoro Mountains, and you'll eventually come across a Free Lancer that keeps throwing rocks at you! Stand there and let it throw rocks, and Frog will catch one and it'll be the Gold Rock!

PrismSpecs
You may know this, you may not...but I didn't for a long while. The PrismSpecs not only make your regular attacks stronger, but all of your Magic becomes much stronger as well.

Crisis Arm
You get a Crisis Arm after beating Geno Dome, but it sucks, right? Wrong. According to Zlefin, the attack power is directly proportional to the "ones value" in Robo's current HP. In other words, the higher the last digit in his HP, the more damage will be inflicted (if the last digit is a zero, there'll be no damage)...

Magus's Best Stuff
I'm sure you already know this, but in Ozzie's Fort, in the room where the poor imp gets sliced and diced by Ozzie, if you go down into the "black area" in the lower right part of the screen, you'll come to an area offering the best stuff for Magus, and a Magic Tab.

Save Lara!
When Lucca's having her flashback about her mommy's crippling accident, you have a chance to right this wrong once and for all. If you go in the kitchen, you'll find a piece of paper saying that Taban made the password of his machine to be the name of his (ahem) loving wife. Now we all knew his wife's name is Lara, right? If not, you may be in trouble. Anyway...examine the machine and it'll prompt you for a password...but...there's no keyboard...how will you...ah yes, L, A, R, and A are all button names! Enter L-A-R-A on your controller, and the disaster will be averted! (for the PSX version, the code is L1, O, R1, O)

New Game+
EVERYBODY knows this, but I'll say it anyway beat the game and get the "normal ending" (you'll also need to make it through the Black Omen). Then you'll get the New Game+ option, where you can start a new game with an old game's items, experience, gold, techs, etc.

The Chair Sitters
Nothing too special, but you can sit in Azala's chair in the Giant's Claw, and your lead character will laugh.

Lots o' Tech Points
An easy way to build up Tech Points is to go to the Mountain of Woe, defeat the Rubbles there, return to the village, and repeat. Each Rubble is worth 100 TP a pop, so you'll be rolling in the techs in no time!

Lots o' Experience Points
If you want lots of experience, go through the Black Omen in 1000 AD, then 600 AD, then 12000 BC, for thrice the excitement, thrice the fun, and thrice the rewards.

Ruby Armor
Visit Ioka village in 65,000,000 BC after the Ocean Palace, and you'll have an oportunity to buy the Ruby Armor for the low, low price of just 10 Feathers, 10 Horns, 10 Fangs, and 10 Petals!