Donkey Kong 64/Hideout Helm

In the room before the Blast-o-Matic with the tube and the Tiny barrel, you can enter the tube as Tiny without using Mini Monkey by doing a well-timed Ponytail Twirl into the pipe. Video (#3)
In the room with the Battle Crown door, there is a computer that's to your right as you face the steering wheel. It's near a klaptrap. By jumping onto the edge of it, you can fall through. You can then walk out of bounds, jump into the room behind the coin door, and get the key. Sometimes, however, the key doesn't count in your inventory. It is believed that by walking through the area that contains the coin door, you "activate" the key so that it counts in your inventory if you pick it up. Walking under the coin door seems to achieve the same effect. Video (#13)
If you use a Gameshark or something similar to shut off the power in Diddy's room without shutting off the power elsewhere, the game will think that you shut down the power everywhere. Thus, a way into Diddy's room without shutting off the power in the preceding rooms would save a great deal of time.