Unlimited Moogle Points

For this secret open two connecting rooms, and wipe out all the enemies inside. The places were moogle points are regenerate after you leave a room and return, but the enemies don't. Just go back and forth in those two rooms to get all the moogle points you want. I find that you get about 2000 in about 5 minutes or so, depending upon how lucky you are.

Contributed by: BoboBobThe3rd 

Ultima Weapon

Beat Reverse Rebirth mode to unlock Ultma Weapons for Sora. (You can buy them in Moogle Shops)

Contributed by: santanian 

Heartless battles without Map Cards.

In order to have unlimited battles with Heartless without wasting your Map Cards simply save your game. Then use a False Bounty and open the 2 chests that contain Heartless but always leave the one with the treasure close. Once you've fought the 2 Heartless chests, get out of the room and re-enter, both Heartless chests will be close again for you to fight them again.
Repeat this process as many times as you want to level up without using any extra Map Cards.

Contributed by: Kingdom_Hearts2 

Premium All Your Cards Fast (or semi-fast)

Similar to the Moogle Points trick, you'll walk between two rooms over and over, and this trick depends a lot on luck.

The requirements are that one of the rooms is a Premium Room, and that that room has to have a BARREL in it. So, once you've cleared the room of enemies, just leave and come back. The barrel will respawn each time, and whenever you break it, you will either get health, moogle points, or, if your lucky, a barrel spider. Get it? The spiders respawn (with luck) and thus your chances of getting a Premium Bonus are high. Even when you don't get spiders, this will help you get Moogle Points.

This certainly takes a while, and I wouldn't recommend trying to do it to your whole collection. When I did it it took me about 30-45 minutes to premium about 20-25 cards. That's pretty good, and it helped a ton. I also got a few thousand moogle points, so that helped to.

I recommend making a deck of high-CP cards that you commonly use, especially stuff like Cloud. Barrel Spiders all have an attack power of 4, so keep all your cards above that and you won't have any trouble. It's tedious, but it really can be worth it.

Contributed by: nmaster64 

Faster Unlimited Heartless Battles Without Map Cards

If you do not wish to(or cannot) use False Bounty, then try this. Warp to the entrance room of a world from a different floor, and there will be Heartless there. Defeat them, then warp to a different floor. You may repeat this as many times as desired without using a single Map Card.

Contributed by: DivineDragoonKain 

Fight Vexen Multiple Times

As soon as you hit The 100 Acre Woods, ignore all the tasks and Pooh and head towards the end. There will be a short cutscene, after that it'll ask you to Save your game. Select NO, and continue on to a boss fight again Vexen. After the battle, you'll find yourself back in The 100 Acre Woods. Complete one task, and head to the end again. You'll find yourself battling Vexen again. After the battle you'll see that you gained EXP from the first battle against Vexen and the second one. Rinse, Lather, and Repeat.

Contributed by: love_hatefully 

Skip pre-boss fight conversations

If you've lost to a boss fight and get tired of listening to the conversations that take place before the fight press start and hold it down for a few seconds and the battle will begin. This becomes very helpful in tough boss fights where you are constantly restarting over and over

Contributed by: churchpunk872 

Link Mode

Contributed by: Izzyzy 

Alternate Title Screen

Contributed by: Typh 

Unlockable cards

Complete the task in order to get the reffered card.

Contributed by: Kionijohn, noz3r0, XxThunderxX, Netmonmatt, TheGame16us, Mykas0, Ragna0k0nline, 1guynamedchris, MechaFox, ReneGadeGirl, SolidSnake754, Vampric Saviour, DivineDragoonKain 

Boss Cards for Riku.

In Riku's Story Mode, your Red attack cards are lost and replaced with new ones from level to level, but you can also obtain Black cards that don't disappear. To use them, press Select during a battle.

Contributed by: XxThunderxX 

Special Cards

Contributed by: NekoLelouch, gotryce22, KeronSHB, goodbling, Zirix 

Unlockables

Recieving the "Gold Card" and "Platinum Card"

Contributed by: drakeswarm 

Reverse/Rebirth Mode

To get Reverse/Rebirth Mode where you play through the story as Riku, simply beat the game once as Sora.

Contributed by: Shinarouji 

Theatre Mode

Beat the game once to unlock Theatre Mode, an extra option on the main menu wherein you may re-watch any cutscene in the game.

Contributed by: KeybladeMasterA