Super Mario Bros 2

99 Lives
Go to World 3-3. Climb the ladder then jump over the spark enemies to the door over on the ledge. Find the red grass patches; the last one is a shell. Pick it up. Go back to the bottom of the area, and then toss the shell into the doorway. Stand on the ledge and wait. After the shell defeats enough Shy Guys to earn 1,600 points, you'll start earning extra lives. You can easily, and quickly, earn up to 99 lives.

Bald Birdo
When you get to the green Birdos jump on her head and press B to pick her bow up. Birdo will be bald!!! If you throw the bow back at Birdo it will land back on her head.

Free Hearts
If you're ever in need of hearts, look for one of those giant Shy-Guy enemies. Jump on it, pick it up, then throw it on the ground again. A heart will come out for you to collect. You can do this an infinite number of times to the same giant Shy-Guy, too

Gain an extra life in each level
To gain an extra life in any level, collect all 5 Red Coins scattered throughout each level.

Warp to World 4-1
In world 1-3 after you get the magic bottle/potion go all the way to the right until you reach the vase thing. Use the magic bottle/potion, jump on the vase and go down. Instead of not being able to go down you'll now be warped to world 4-1.

Warp to World 5
Go to level 3-1. Go to the waterfall and go down. Keep going until you reach the hill, go in the door on the hill. Pull up veggies until you get a potion. Go to the right and place the potion next to the pot. Enter subspace and go into the pot. You will be warped to the 5th world.

Warp To World 6
In the begining of level 4-2 jump down onto the whale and go to the left. You will find some red grass patches which will have a potion. Carry the potion through the entire stage, and rather than go through the door, you can jump on the last whale's tail and jump over a small gap to find a vase. Now drop the potion, enter the door and go down the vase.

Warp to World 7-1
Go to world 5-3 as Luigi. After you climb the first ladder, power jump (hold down until you are glowing) on to the platform you normally can't reach. Pull up the plant on the right side of the warp pot. Throw down the bottle and go through the door. In the dark world, jump on top of the pot and go down. You should see the screen wiggle and it will say warp world 7 on a screen with Birdo.