runouw.com

12/24/11
You can walk a slight distance before connecting a double or triple-jump. This can be useful for making tricky ice jumps some jerk happens to come up with *cough*CoDt*cough*
12/25/11
Ground-pounding will not only break blocks below you, but also around you. You can use this as a trick to conquer, or build, ice mazes.
12/27/11
Ground-pounding can completely negate fall damage. If you are in a bottomless pit, it can also speed up your death.
12/28/11
Luigi can be accessed by collecting 32 Star Coins. He can jump higher and run faster than Mario, but often switches to Mario's sprite during cutscenes.
12/29/11
Using the Search option in Microsoft Word can help find specific items in your level's code, like signs or items.

1/1/12
Spinning after using the Turbo Fludd can yield farther distance if done right.
1/2/12
Spinning in water can keep you afloat.
1/5/12
Pressing down while in the air repeatedly can let Mario travel at VERY high speeds.
1/6/12
Backflipping is used by pressing up and backward while holding down. This can stop you on a dime and can help make tricky jumps.
1/7/12
To disable an item, put a 0 in front of the item's ID. For example, a disabled Shine Sprite is 06, and a normal one is 6.

1/8/12
Double-tapping forward allows Mario to sprint. Not that useful, but fun.
[spoiler]1/9/12
Offset can make an object start operating sooner or later than surrounding objects. This can be good for making artificial danmaku.