Some nice ones in the PSX version of FFT:

If a character gets covered with Oil, they're supposed to be vunerable to fire damage, but apparently they're not.

The weapon duplication glitch allows you to copy swords, knight swords, katanas and the like using the fitting room at shops. That's fairly well known.

You can use Degenerator traps to decrease levels. While this sounds bad, if you do it while in a class with poor stat growth and level up again in a class with good stat growth you can maximize several stats. Again, it's common knowledge.