8-bit Mario and partners
In X-Naut Fortress, go in to the ventilation shaft in sublevel 2 and drop through the far right vent in to the dressing room. Open the curtain of the stall you fall in to and Mario and all his partners will become 8-bit (they will look as they would on an 8-bit system like the NES) until you leave the room. Entering the room the normal way via the door and going in to this stall, closing the curtain, and opening it again will not work.
Contributed By: Alastor the Stylish

Angered Rawk Hawk
After you sign into the Glitz Pit and win a few fights, come into the main lobby to see Rawk Hawk being adored by his fans. Use Mario's Hammer or Flurrie to get rid of the fans, and when all of them are gone, Rawk Hawk will get mad and scold you.
Contributed By: baconlabs

Lottery Cheat
In the lottery at Rogueport if you change your game cube clock to the next day you can cheat the lottery. The second time however you are fined 500 coins for cheating.
Contributed By: V i v i

Mario called "Jumpman" by bystander
In Glitzville, when you make a "comeback" after you beat Rawk Hawk the first time and fight your way to the finals, one of the spectators will, instead of cheering "Gonzales", or something to that effect, will cheer "Jumpman! I mean, who?" Jumpman was the original name for Mario before he was called Mario, IE back in the days of Donkey Kong Arcade.
Contributed By: Crank Yanker 606

Showering Peach
As Princess Peach (this can be done anytime you can take control of her), go to the room left of your bedroom and go up to the shower. When an exclemation Mark appears press A and Peach will take a shower! After she comes out she will also have changed her hair style!
Contributed By: barkera

Super Mario World music
When you get an important e-mail (ie; the e-mails from Princess Peach), wait a couple of seconds and you will hear music from Super Mario World.
Contributed By: GreatGonzales

Atomic Boo and Lucky Start
In Creepy Steeple, after you've freed the Boos, stand still in the main room.
Wait for some Boos to start sticking to you, the use your Super Hammer attack to shake them off. Do this twice, and the Boos will get mad and form the Atomic Boo. When beaten, he will drop the Lucky Start badge. This is needed if you want to fill up your Tattle Log and get all badges.
Contributed By: baconlabs

Easy Coins
Sleep at the Inn 50 times and a reporter (a lakitu) comes and rewards you with 200 coins.
Contributed By: mariopaper

Easy Money
Buy as many sleepy sheep as you can carry at the Toad Bros. shop and sell them all at the Petalburg shop for a 2 coin profit each.
Contributed By: the cool

Ghost's Curse
This really won't help you in your game, in fact, save before you do this, or you'll lose some game data. -- When you find the Ragged Diary in the Excess Train's Baggage Cart after meeting the ghost, read the diary and the ghost will appear out of nowhere really angry and curse Mario, transforming Mario into a dead spirit, earning you an instant game over.
Contributed By: OmegaX

Look at dialogue that you accidentally missed.
While someone is talking, you can press Z to go back through the dialogue. Grey dialogue means that it has already been read. Black is normal.
Contributed By: EvilDarkLink

Missing Tattle Logs
Missed any Tattle Logs, particularly boss logs? They are not gone for good. Check Professor Frankly's yellow trash can. You will find Tattle Logs thrown away for bosses and other once-only enemies. The log will only appear in the trash once you've gone past the last possible point to Tattle that enemy.
Contributed By: Elranzer

Poison Shroom Heal you to Max
If you make a Poison Shroom from Zess T. (a Slow Shroom and an Inky Sauce), the effect will poison you. If you have the Feeling Fine badge on, however, it may do one of 2 things:
1) Heal you to max
2) Cut your HP in half
Be wary when doing this.
Contributed By: blaiken used peck

Waluigi's Clothes
Get both the W emblem and L emblem and put them on, Mario's hat and shirt will be purple and his overalls will be purple, just like Waluigi.
Contributed By: Crystalking52