Adamant Armor:
Use Alerts in the L-Shaped room in the Lunar Underground 5th floor to summon Pink Puffs. There is a 1/64 chance that they will drop a Pink Tail. Take this tail to the Adamant Grotto for Adamant, then head for the blacksmith's house, located underground, to get the Adamant Armor.

Developer's Room:
Yes, there is a hidden Developers' Room, and it's in the hardtype version of FFIV, so here's how you get to it!

Go to the Dwarven Castle in the underworld. Get to the room where you can buy weapons and armor; there is a path in between both those counters. Walk up into the Rally-ho Pub. There, walk directly to the right (around the table of course) and you'll see an odd blackish section of floor. Walk right through the wall at that point and continue right and down the steps into the hidden Developers' Room!

There is also a hidden porn magazine in this room as well. Get into the Nap Room (from the entrance to the room, go up and right, then down into the Nap Room). Check out the left side of the bookcase nearby to find Smut, which is the magazine. Select it on the Item menu and the screen will turn purplish as Cecil looks at it... Then after it's over, the screen's back to normal and the Smut is gone.

Item Duplication:
You will only only be able to dupicate items that can be held in the Left Hand or Right Hand. A great way to get loads of Excaliburs to throw. Go into battle. Now select an empty slot in your item list. Now keep pressing UP until you get to the equipment list for the Left and Right Hand. Select the item that you want to duplicate. Now escape from the battle by holding L+R. Open up the menu and go to the character equipment. Re-equip your item. You will notice it has a "2" next to it. De-Equip it and then Re-equip it, to "split it in half."