There are two types of games that relate to this question for cheat codes. 1. Animal Crossing Calculator 2. Animal Crossing Wild World.
Animal Crossing Calculator cheats
If your sum contains only sequential digits (12345), when you press "=" Tom Nook will make a surprise appearance. If your sum contains all of the same digit (77777) then the effect will be the same, but with flowers instead of Nook. Sadly, it's still merely a calculator.
Animal Crossing Wild World cheats
Unlock Blue Feather: You must donate 200,000 Bells in Boondox.
Unlock Green Feather: You must donate 10,000 Bells.
Unlock Mush TV: Go to the Acorn Festival and collect 140 Acorns.
Unlock White Feather: (rare) You must donate 1,400,000 bells
Unlock Museum Model: You must be the roommate that donates the most amount of items to the museum
Unlock Academy paper: You must sign up with the HRA, and your scores shall arrive on academy paper.
Unlock Red Feather: You must donate 800,000 bells.
Unlock Modly Shirt: Check with Gracie. She will attach a moldy shirt to her evaluation of your fashion trend.
Unlock Yellow Feather: You must donate 500,000 Bells.
Unlock Golden Shovel: Bury a shovel anywhere and dig it up one day (24 hours) later. The shovel will become a Golden Shovel. Using the Golden Shovel, you can bury bags of money to hopefully grow money trees. Larger lumps of money increase the odds of growing a fruitful money tree.
Unlock Fruit Tree: If you've got some stuff you don't want, like ugly furniture or shirts, you can attach it to a letter and send the letter to another resident of your town. Town residents will almost always respond with a letter of their own and a different item attached.
Unlock Rock Money:
In town, you may attempt to hit each rock with a tool to shake it out for money. You may continue your shake down until this rock exhausts itself. This manner of money-making may be done once per day. When you hit the rock, money will come out. However, you will bounce back. You will have to run back to hit it again. To prevent this, dig holes around you in a pattern so that you won't bounce away. You can make 20,000+ bells a day this way, but you have to be fast to get this much.