Item Duplication:
You can make 255 copies of any item! 255 Apples (raise your stats), 255 Duplicators (no more Hayokontons!), 255 Crest Graphs (get all the spells right at the start), 255 Secret Signs (all your Fast Draws cost 1 MP), 255 Ambrosias (infinite healing), etc. Best of all, it's a simple glitch and works every time...

You must have one and only one of the item you want to duplicate. Go into a battle and have your first two people use heal berries on themselves. Have the third person switch your heal berries with the item wanted to be duplicated, and voila, 255 of that item at the end of the battle. Go nuts.

The Dog:
Go to Surf Village and bomb or punch the dog. It will go flying into the air, bounce a couple times, then chase after you! Useless, but interesting.

Good Stuff:
Want a Power apple, a Hardy Apple, an Agile Apple, a Mystic Apple, and a few potion berries? Then listen up. Go to the Ancient Arena and fight Medea and Badnews, then stop. You will get the items listed.