Mario Kart 8 (WiiU)

Rocket start
During the starting line countdown, press and hold the accelerate button a split-second after the second light turns on to start the race with a short burst of speed.
Disappearing ink
When your screen is gooped up with Blooper ink, use a Mushroom item or drive over a dash panel to quickly clear your screen.
Gold trophy stars

In Grand Prix mode, you can get up to three stars on your gold trophy depending on your point total after all four races. Three stars is the best rank and requires all first-place finishes (60 points). Just one second-place finish drops you down to two stars (57 points). Finishing with 55 or 54 points will give you one star. Anything under 54 points results in zero stars.

Unlock Miiverse stamps

Out of the 90 total stamps in Mario Kart 8, 28 are available to use right away. The rest are unlocked one at a time in the Grand Prix and Time Trial modes. Each racer has a stamp that you can unlock by getting a gold trophy with him or her in Grand Prix mode. In Time Trial mode, finish a course with a faster time than the staff ghost to unlock that course's stamp.

(Note: Version 4.0 adds 10 more stamps that are also ready to use right away.)

Unlock new racers
Every new gold trophy you get in Grand Prix mode will unlock a random character, except when unlocking the Mii (see below). There are 14 hidden racers in total.
Unlock Mii
The eighth racer you unlock will be your Mii.
Unlock Mirror class

Earn a gold trophy in every 150cc cup to unlock the Mirror class.

(Note: In Version 4.1, Mirror class is unlocked from the start.)

Unlock Gold Standard kart
To unlock the Gold Standard kart, earn at least a one-star rank in every 150cc and Mirror cup.