Unlockable: Get Yojimbo
Go to the cavern of the stolen faith and you will encounter a dead summoner who calls Yojimbo. Beat him and continue on to the teleport pad. Choose to travel to the chamber of Fyth and Yojimbo will ask you why you seek his services. You must answer, "To defeat the most powerful of enemies."
Yojimbo will then ask 250,000 gil for his services. Offer him half of that amount but add one gil more. So if he asks 250,000 offer 125,001 instead. Then do the same until he asks 205,000. When he asks 205,000 offer 200,000 and he will most likely accept the offer. You can offer the full 205,000 gil to hire him for sure, but if you don't mind losing an extra 5,000 gil.
Unlockable: Magus Sisters
Capture all the required fiends on Mt. Gagazet then return to the monster arena owner in the Calm Lands. As a reward, the arena owner gives you the BLOSSOM CROWN. You must also possess all of the other aeons, including Yojimbo and Anima.
Next, go to Remiem Temple in the Calm Lands. After you defeat Spathi, Belgemine will give you the FLOWER SCEPTER. After defeating all the aeons at Remeim Temple, use the Blossom Crown and Flower Scepter on the door in the back to unseal it and acquire the Magus Sisters.

Airship Passwords
You can also input passwords to uncover secret locations. These passwords are hidden throughout the game in Al Bhed -- but if you can't read it, here they are (note that they have to be entered in all caps):


GODHAND -- Let's you go to lower Mushroom Rock Road where you will find Rikku's legendary weapon.
VICTORIOUS -- Enter this to access a secret area in the ruins of Besaid. Climb the tree to get your hands on Rikku's powerful Victorious armor.
MURASAME -- Another secret area in the ruins of Besaid. You will find Auron's Murasame (ability: One MP Cost) here.