Mario Kart: Double Dash!! (GCN)

Time Trial passwords
On the "Retry / Change Course / ..." screen after a time trial race, press L R L R X Y X Y Z and a password will pop up at the bottom. So far, Nintendo has only used the passwords for a contest in Japan.
Let's-a go, already
Lakitu's fishing intro can be skipped by pressing Reset.
Rocket start
Press (and hold) the A button right when Lakitu's light turns green for a short burst of speed off the starting line. In two-player co-op mode, if both players get a rocket start, the result is a bigger burst of speed called a "double dash."
Lakitu boost
To quickly get back up to speed after falling off the course, press (and hold) the A button right as Lakitu places your kart back on the track. If your timing is true, you'll get a short burst of speed.