The Hidden Blade Assassin's Creed in Depth

This relies on two exploits. In case you're not familiar with them, the first, Skipping Through the game is an exploit discovered by hit468 on the Xbox360 where you replay a memory, then sign out of your account, sign back in, and go on a different save file. When you start you will be in the same replayed memory, and while you are in replay the game thinks you're that far in the game regardless of how far you actually are, even if you just started a new game.
http://thehiddenblade.com/assassins-creed-2-temporarily-skip-far-game

The second is the fast travel glitch. Discovered by IanXO4, you can fast travel after finishing a replayed memory before the game reloads the current memory. Combined with Skipping, you can visit any area to collect items and stuff, which makes my video possible.
http://thehiddenblade.com/mark-and-execute-assassination-variety-pack

Auditore Cape

Some time ago I use these exploits to collect all one hundred feathers and obtain the Auditore cape (one of the last items you can unlock in the game) very early. But after exiting replay the cape was removed from my inventory. It was before Ezio had the assassin outfit. When I went back in replay and went to the villa it played a glitched scene with an invisible Ezio's mother.
http://thehiddenblade.com/maria-disappeared-permanent-crash
I thought the game would forever be broken, but decided to play through as usual. When first reaching the villa the scene for the fifty feathers plays, desynching Ezio. Luckily it doesn't play again until you get close enough to the villa.

The scene for one hundred feathers doesn't play until after talking to Ezio's mother and sister in the villa. The scene starts shortly after that conversation, so try to reach the room with the feather chest before that happens. If done right the scene will play as usual with Ezio obtaining the Auditore cape in sequence three. If you don't get close enough it plays the glitched scene, breaking the game. Don't do what I did but instead back up a save before depositing the feathers.

I also have a theory that if you get the cape right after getting the assassin outfit you won't lose it, but I've already spent too much time on this.

Codex Pages

It was suggested you could use these exploits to get codex pages, and therefore more health, early. If I remember right here's how you get all of them.
1. In replay from a late save you automatically have a few from the storyline.
2. You get over half from chests inside cities.
3. There are already six on the codex wall.
4. You get the last five shortly after killing Vieri. Mario gives you one right after that, and after talking to Mario about the codex pages you get four from chests in the villa.

Now the matter of deciphering them. While in replay you can't talk to Leonardo, but there are times early in the game you can. When Leonardo fixes your hidden blade in sequence two all codex pages you currently have are decoded. The most you can have at that time are nineteen, which amount to four squares of health. The last five will give you one square of health.

Altair's Armor

I originally thought it would be impossible to get all six seals early in the game because one of them is obtained in the story. But then I remembered the game will give you items obtained throughout the story of you're replaying from a save that far in the game, so I did that, and lo and behold in the inventory there was one seal from the storyline. The other seals I got from replaying the other assassin tombs.

When you unlock Altair's armor that early it gets glitched in some ways. First I was able to put all but one of the seals in place. After fast traveling and coming back I was able to put the last seal in (more than once because replaying multiple times gives you more than one of the same seal). So I expected to be able to equip the Armor of Altair when reaching the villa. But then it wasn't available on the armor mannequin or in its usual spot behind the bars. And here's the strange thing: if I went back into replay from a late save Ezio would be equipping armor, which meant it must become unlocked sometime in the game.

Another glitch caused by having Altair's armor unequipped early in the game is an invisibility glitch with the hidden blades. The blades and the bracers containing them can't be seen. Later in the game one of them becomes visible. Also you can still do double assassinations but do the counter for one hidden blade. The animations for the single hidden blade counter were one thing I missed after getting the second hidden blade, so I guess this exploit wasn't a complete waste.

Altair's armor automatically equips at the beginning of sequence nine when the last assassin tomb becomes available. That makes trying to get it earlier useless. Also the second hidden blade becomes visible once again even if you unequip the armor.