Perhaps the biggest cut in Final Fantasy Tactics comes in the form of six battle stages hidden in the games code. Three of these are complete and fully playable (via gameshark): Hospital in Slums, Warjilis Trade City, and Gate of Limberry Castle—though each room in the hospital can only be seen from a specific angle. The other three battlefields—Inside Castle Gate at Lesalia, Outside Castle Gate at Lesalia, and Main Street of Lesalia—are somewhat playable but there are glitches which make the wrong movement and attack range information appear. Also hidden in the code are four maps which appear to be prototype maps to test the game's battle system.
There are 20 normal job classes in Final Fantasy Tactics (as well as an additional nine special classes), however the game's code hides an additional nine jobs on top of those that are never seen in the game proper. These classes are: Cardinal, Duke (Goltanna), Duke (Larg), Grand Duke, Elder, False Saint, High Confessor, Viscount, and Witch of the Coven. Basically, every character in the game has a job, regardless of whether they ever enter battle or not. So, these classes don't have any skills in them, but oddly, they do each have a description for each of the classes.
There is also one giant piece of content cut from only the Western versions (both PS1 and PSP) of the game: The Sound Novels. These are a set of four choose-your-own-adventure-style side quests. They are not part of the main story, but they do flesh out the world of Ivalice. (They can be found fan translated into English here).
In addition to the maps, jobs, and sound novels mentioned above, there are also dummied skill sets, abilities, and several unused enemy/item/character sprites—including one character spite complete with portrait.
Frankly there's a lot of content hidden on the Final Fantasy Tactics disk. And getting to see it all these years later is an interesting look into the design process of it and other similar games. To see the full list of dummied content in Final Fantasy Tactics check out the full entry at the Final Fantasy Wiki.