Final Fantasy 3

# Secret dungeon

Send seven mails to another player using Mog Net. Then, send four mails through Mog Net to the NPC "The 4 Old Men", and another four mails to "King Alus". It does not matter what order this is done, as long as Alus is last. After reading the fourth one you will unlock the new dungeon. Ride on the Nautulus and go east of the floating continent. Once there you will see a shadow in the water. Go underwater and move into the coral to enter a cave. There is no real dungeon here, but there is a difficult Boss, Iron Giant.
# Onion Swordsman job

Register someone multiple communication IDs on Mog Net. Send seven messages to another player. Send and receive four messages from Topapa. After reading the fourth one, you will get a message giving you a mini-quest. Go to Uoul and talk to Topapa. Go to the cave that is to the north of the other cave. Enter, then defeat the three Bombs to get a piece of crystal. This will unlock the Onion Swordsman as a job.
# Item duplication

This trick works on any item you can use in battle, such as Elixirs or Phoenix Downs. Enter the item menu outside of battle. Move the item to be duplicated to the first slot. While in battle, choose the item menu, then press Up + A. If done correctly, you should see a flash of the equipment screen before seeing the main menu again. Press B to cancel. The item is now duplicated. You can repeat this as many times as desired, but be careful not to get more than 99 of an item as it may glitch your game or saved game file. Note: Items can be in either the top left or top right hand corner to be duplicated.
# Easy Gil

Choose a high-priced item to sell, then use the "Item duplication" trick. Sell all of them except one, then repeat as many times as desired.

Go to the Town Of The Ancients and talk to the child two stories up the grass hills. He is back behind the stairs. He will tell you that he will give you a prize if you ride around the continent. Leave town and go east a bit. Get a Chocobo and ride around the outside of the continent. Go back and talk to the child to get your rewards. Sell them, then repeat.