Eragon

Fury mode:
Press Start to pause game play, then hold L + R and press X(2), B(2).
fysfapami.

Full health:
Press Start to pause game play, then hold L + R and press X(4), B(2). Your health will go up to full. Repeat this as many time as desired.

Hint: Defeating Durza:
There are two stages to fighting the evil Shade. For the first stage he will teleport around, slashing at you with his claws and shooting streams of flame. He will charge up his fire attack. When he does, run up to him and press A or B to grab him in a "full nelson" move. Keep tapping A, B, X, and Y and Brom will come over and hit him with his sword, Zar'roc. The Shade will break free. Hit him with your sword when he gets close. Get him to half health and the second stage of the battle will begin. In the second stage he will continue to teleport around, but will also ditch the fire stream attack for another. He will float in the air with a large red bubble shield around him. Many scimitars will fly off the walls and will shoot through the air towards you. After they stop, run up and attack him.
Tyler Christenson.

As you start the final battle with Durza, hold R + L and shoot Fire Arrows at him on his demonic steed. When he fires purple missiles at you, release R and keep L held. Press A rapidly, and Eragon will deflect the missiles back at Durza. However, when the Shade fires Fire Balls, you cannot deflect them. Just dodge them. Continue this until he is dead. There is a secret egg in this level, but is extremely difficult to find. Look for the telltale blue dot on the screen.
Tyler Christenson.

Use the following trick when facing Durza at the final battle. You will have to fly Saphira while trying to defeat Durza on The Shadow of Tirani. To defeat him, shoot your magic arrows at Durza. After he says something similar to "You should have taken my offer boy", Press the L and prepare to press A to shoot back the dark magic. However, he may shoot fireballs. To dodge these, fly in circles when they get close to you. After awhile you will defeat him. There will be a scene with Durza throwing a spear at Eragon. He will somehow disappear and stab Durza through the heart. Durza will die and Ajihad will tell the rest.
howell8577.

Hint: Defeating Durza:
When you reach Durza, continuously hold [Block] and use the fire move against him. However you will still have to dodge when he is about to grab you.
Kellie Frazier.

Hint: Using Thrysta Vindr and Brisingr at the same time:
You will get many powers and moves. With this trick, you will only have to worry about the magic. Press L, then press A + Y at the same time. You should burn with "Brisingr" and push with "Thrysta Vindr". This is useful when fighting the King's soldiers.
shane.

Hint: Hidden egg bonuses:
Collect the hidden eggs found throughout the game to unlock the following bonuses.

1 egg: Ad for the Eragon and Eldest books
2 eggs: Edward John Speleers commentary
3 eggs: Design commentary
4 eggs: Art direction commentary
5 eggs: Birth Of A Game commentary
6 eggs: Concept art
7 eggs: Orchestra commentary
8 eggs: Saphira commentary
9 eggs: Sienna Guillory commentary
10 eggs: Character concept art
11 eggs: Concept art
12 eggs: Concept art
13 eggs: Robert Carlyle commentary
14 eggs: Sound Effects commentary
15 eggs: Garrett Hedlund commentary
16 eggs: Evolution Of Games commentary
17 eggs: Concept art
18 eggs: Throne Room bonus level

Glitch: Saphira goes on rampage:
On the level where you and Murtagh are in the water and ogres are everywhere but Saphira is helping you, keep telling her to go to the same place constantly. Eventually Saphira will go on an attack rampage. She will go back and forth from side to side killing everything, and you can just sit there and watch.
Gabriel Fraenkel.