The Legend of Zelda: Ocarina of Time/Secrets

Giant enemies
If you repeatedly kill some types of enemy within a short period a giant version will appear. This applies most to the Stalchildren running rampant during the night on Hyrule field.

Never lose tunics and shields
When a Like-Like is eating you swap to the Kokiri Tunic and Mirror Shield. Because neither can be purchased nothing in the environment is able to destroy them, and so the Like-Like can't eat them.

Free money
As young Link, travel to the gate in front of the city. Stand on either chain, and wait until night. The gate will be raised. Walk across the narrow end of the upturned gate to collect 80 rupees (four red ones). This trick can also be done during the day time, simply walk straight up the chain until your face is in the wall then turn towards the middle and jump down. You'll get 20 rupees while you're falling and enter Hyrule Castle Town.

500 Rupees
This trick requires Adult Link, the Biggoron's Sword, and (preferably, although not required) the Giant's Wallet. Enter the Lost Woods and turn left. There should be a Skull Kid standing on top of a tree stump. Approach him, and when he advances on you, quickly attack him with the Biggoron's Sword, using the stabbing-motion (forward+B). If you are able to defeat him before he disappears, you'll be rewarded with a special rupee, so big that it will fill any wallet you have, even the Giant's!

A similar reward can be achieved with the help of the Bunny Hood, which Young Link can obtain from the Happy Mask Shop once he has already sold the Keaton Mask, Skull Mask, and Spooky Mask. Once you've got the Bunny Hood, go to Hyrule Field, approaching Lon Lon Ranch. You should eventually see a man running laps around the ranch. If you wear the Bunny Hood and speak to him, he will ask you to sell it to him. Say yes and he'll pay you five hundred Rupees. (it is preferable that you have the Giant's Wallet for this)