High Definition mode
Before the “Sony Computer Entertainment Presents” screen appears, hold L1 + L2 + L3 + Square + Circle. If you entered the code correctly, the letters on that screen will turn purple. -From: Larry Solinger

Challenge Of The Titans mode
Successfully complete the game to unlock the Challenge Of The Titans mode.

Arena Of The Fates mode
Get a “Titan” rank in Challenge mode to unlock the Arena Of The Fates mode.

Titan mode
Successfully complete the game to unlock the Titan difficulty setting. (more…)

Unlockables
Successfully complete the indicated task to unlock the corresponding bonus the next time a new game is started. Once the game is completed, the “New Game +” option is also unlocked, which allows you to use your existing character with previously earned equipment and skills.

+10% Damage Reduction: Get the “Soldier Ally” achievement.
+10% Damping and Overload reduced cooldown: Have a 75% game completion with Garrus in your squad.
+10% Experience: Reach level 60.
+10% Hardening: Have a 75% game completion with Ashley in your squad.
+10% Health: Kill 150 Organic enemies.
+10% Lift and Throw reduced cooldown: Have a 75% game completion with Kaiden in your squad.
+10% Sabotage and AI Hacking reduced cooldown: Have a 75% game completion with Tali in your squad.
+10% Shield: Kill 250 Synthetic enemies. (more…)

Play as Luigi
Collect all 120 Stars. Then, go to Rosalina on the Comet Observatory, and request to fight Bowser again. Defeat Bowser to view an alternate ending. Once the credits finish, you will get a message saying Luigi is now playable. Luigi is just like Mario, but he jumps higher, runs faster, and has lower traction.

Grand Finale Galaxy
Collect all 120 Stars with Luigi, and defeat Bowser to unlock the Grand Finale Galaxy for both Mario and Luigi, which is the Star Festival seen in the beginning of the game.

Star 121
Collect 100 Purple Coins in the Grand Finale Galaxy to get the final Star. (more…)