Super Mario Kart (SNES)
In Time Trial, finish a course, and get a ghost. To save it, hold L, R, Y and press X. You should hear a coin sound. The track name will turn orange. To race the ghost, hold L, R, and press start on the "Is this OK?" screen.
To erase all the best times and GP data, go the the title screen and press L, R, Y, and A simultaneously. If you want to erase the times only, use the same code on each of the Time Trial runs.
On the Player Select screen, highlight your driver and press Y and A simultaneously. The only way to change back during a race is to run over a Mushroom dropped by the Princess. Note: This will not work in Time Trials or Battle Mode.
Special Cup Time Trials
Here's how to access the Special Cup tracks in Time Trial mode early: Enter the Time Trials and select your racer. Then push Left on your controller to move the cursor beside "Mushroom Cup." Then press L, R, L, R, L, L, R, R, and A. You should see "Special Cup" appear below "Star Cup."
Race Ghost in Time Trials
On Controller 1, begin a one-player Time Trial. When you come to the driver select screen, press Start on Controller 2. Select a driver with Controller 2, and then select a driver with Controller 1. Begin the race by pressing Start on Controller 1, and you will be racing against a CPU player.
Hold L and R while you select a one-player game in Grand Prix mode. Select your class, driver, and track. When you start GP mode, you will be at the bottom of the screen, in 7th place.
Control the camera in Time Trial replay
In Time Trials, complete a course without hitting a pipe, wall, etc., or falling off the course. Choose "Replay" from the menu. Using the L and R buttons, you can control the camera angle during the replay.
Press and hold the B button a fraction of a second after the first light turns off for a nice boost to start the race. This works in every game mode.
In Ghost Valley 1, get a Feather from the Question Blocks. Then, when you complete a lap, turn around and jump off the track with the Feather, landing behind the finish line (refer to this diagram sent in by $aVaGe DeAtH). Lakitu will tow you out, and then you'll get credit for another lap. You can get some extremely short lap times using this trick! For example, as seen in the third screen shot, we got a 2.84 second lap time -- and that was the very first time we got the trick to work.
A similar, but slower method can help you skip a lap in Vanilla Lake 2. When you cross the finish line, turn around and drive out into the deep water, heading behind the finish line. Lakitu will eventually tow you out and bring you past the finish line again, crediting you for another lap.
Sent in by: $aVaGe DeAtH
Choco Island 1 has a section that appears to be out-of-bounds, but you can drive inside it without being towed out by Lakitu, and items that are thrown inside it will survive. This special area is the large rectangle cut out of the corner straight ahead from the Question Blocks. To get inside, jump over the colored block barrier with a Feather. To get out, drive into the chocolate expanse beyond the edge of the map, and Lakitu will tow you back onto the track.
In the image below, the special area is in the top-left corner of the map.