Super Mario Galaxy Cheats

Unlockable: 121st Star
There's actually 121 Stars in the game, not 120. To get it, you must first have all 120 Stars. This unlocks Luigi. Beat the entire game with Luigi, defeating Bowser at the end. After this, a new galaxy will be unlocked. Finish this final galaxy to get the 121st Star.
Submitted by Molly Metroid

Unlockable: Luigi
Here's how to get Luigi. First, get all 120 Stars in the game. Then, return to Rosalina (on the Comet Observatory) and request to fight Bowser one last time. Defeat him and let the credits roll. Thereafter, a message will appear telling you Luigi is a playable character.
Submitted by Stewie343

Unlockable: Death Count
The Grand Finale Galaxy, which is the secret galaxy that can be unlocked after 120 Stars are gathered with both Mario and Luigi, holds another secret. Once the final Star is acquired with both characters in this galaxy, you will be able to see your death count on your save file. This will allow you to see how many times you died while playing the game with both Mario and Luigi.
Submitted by ScottMartinHall

Unlockable: Additional Demo Stages
If you're playing a demo version of the game at a kiosk, try holding down the 1 and 2 buttons as you press A and B to start the game. This will unlock additional stages for you to play through.
Submitted by liam--9

Hint: Second Player
If you have a second Wii controller, you can have a friend play along with you. While there won't be a second player on-screen, you will have a second cursor to use. This means that Coins, Star Bits, and the like can be collected by the second player. There's also some functionality in controlling Mario's jumps.
Submitted by Andre Segers

Hint: Extra Lives
Because you start each game with only four lives (after shutting the game off and turning it back on), you might be in need of some extra lives upon rebooting the game. You can usually get five right off the bat from Toad (he'll run around with a letter above his head, symbolizing a letter from either Luigi, or more likely Peach). There are four more strewn around the Observatory that you can acquire, however:
1.) On the moving green platform below the core.
2.) Under the stairs leading to the kitchen.
3.) Right at the top of the observatory. (Once the Red Star is unlocked)
4.) Underneath the entrance to the gateway to the stars. (Once the Red Star is unlocked)
5.) Next to the StarShroom (in a crate)
Addition by Cook_Man4eva
Submitted by dark zephyr

Hint: Enemy Sacrifice
This may be an obvious hint, but it's one worth stating nonetheless. When you fight most enemies, you can control what comes from their dead bodies when defeated (a sort of "sacrifice"). By jumping on an enemy, which is usually the more difficult way to kill most foes, a Coin will appear. Spin-killing or using other methods will usually result in a few Star Bits.
Submitted by Pooba Jones

Hint: Infinite 1-Ups
If you grab any 1-Up Mushroom at the Comet Observatory, you can then head into any dome, back out, and arrive back at the Comet Observatory without having played any stage. This will make the 1-Up Mushroom you acquired reappear.